Some documents, publications or books are read from beginning to end
and are never opened again; others are read partially or only
browsed, and then referred to again when looking for more information
on a specific item. Electronic documentation is the ideal format for
this ‘reference’ mode, as in addition to the traditional table of
contents and indexes and their online implementations, it supports
search functions to efficiently locate all instances of required
Text retrieval functions make the difference between a digital
haystack where items are ‘known to exist somewhere’ but nevertheless
cannot be located without a significant effort, and a document
collection where required items can be found instantly even when
there are thousands of pages in multiple files.
Acrobat offers two text retrieval functions that differ in concept
and implementation: the basic Find function, and the more advanced
Search function. The differences between these functions will be discussed in
detail later in this column.
What You See Is Not Necessarily There
Both Find and Search suffer from a number of problems related to text
representation. For both functions to work, text in the electronic document
must be identical to the text in the original document. This may seem
obvious, but many insidious side-effects are introduced when a document is
converted to PDF, and often even the PDF producer, let alone the reader, is
unaware of the implications this has on text retrieval.
Correct text representation, however, should not be
taken for granted, as there are still several issues which will cause
text not be to searchable consistently:
- Older drivers, ATM versions and printer driver settings can cause
text in some fonts to be ‘garbled’ internally (it prints and displays
as intended, but it is not searchable).
- Pre-Acrobat 5 font issues for producers/readers using older versions,
which cause text to be distorted.
- Ligatures (with a few exceptions), small caps and old-style
numerals are not searchable in Acrobat.
- Acrobat may interpret visual white space between letters as
additional space characters. With some fonts, this may happen even
when standard spacing is used. In other cases, spaces between words
may be ignored with smaller-size text.
- Sometimes larger characters may be converted to bitmaps in the
production process (driver-dependent).
- Type 3 fonts (either used in the original document or fonts
transformed to Type 3 during the PDF production), may not be
It is also worthwhile noting that documents scanned into PDF are not
searchable. Scanned PDFs to which optical character recognition (OCR)
was applied may be partially searchable, but with a significant
numbers of errors — with words not found or mis-recognized.
Leaving these issues aside, one has to remember that PDF is
essentially a presentation format and not a document in the sense of
text flow. Searches will not work when the phrase being searched for
is split between pages. Depending on the applications used to author
and create the PDF, there may be additional problems of lines placed
in a reversed or unexpected order, so that phrases split across lines
are not located. Items split between lines in table cells or in
multi-column layouts may also be impossible to locate (as the word
sequence uses a logic different from what is expected).
Hyphenated words or phrases with natural
hyphens split between lines may also pose difficulties, depending on
the specific authoring applications or PDF creators. When creating ‘Tagged PDFs’ (authored with Word2000 + PDFMaker 5 or
FrameMaker 7.0), the additional information stored in the PDF file
significantly improves Find/Search functionality. ‘Structured PDFs’
(as authored with Word98 + PDFMaker or FrameMaker 6.0) make no
difference with respect to text searches,
despite the extra structure information embedded in the PDF.
The Find function (Edit > Find) does not require special preparations
on the part of the PDF producer, other than verifying that text is
interpreted correctly in Acrobat. Find locates the specified text
(word or phrase) in the currently open PDF file only (locally or in a
web browser); options include matching letter case and ‘whole word’.
In terms of speed, the Find function is rather slow, even with the fastest computers. (Try locating a phrase, which is present in one of
the last pages in a PDF file containing a few hundred pages, and you’ll see the status bar showing the page numbers rolling page by
Compared to Find, the Search function (Edit > Search > Query) is much
more efficient. Search supports powerful text retrieval functions
such as looking for multiple words, together with logical operators
(And, Not, Or), with optional Proximity (locating multiple items only
if they are in approximately the same three-page zone, or a larger zone
if there is not much text per page), as well as word stemming,
‘sounds like’, thesaurus and wildcards options.
The Search function can also use PDF metadata, i.e. file-specific
DocInfo fields such as Title, Keywords, Author, Subject (and
optionally custom fields). When including these fields in the search
query, fields and their value range can either be typed directly, or
can be added to the Search dialog box (Preferences, Search, Include
in Query); custom fields can only be typed directly. It is possible
to search based on field values exclusive, or to combine phrases with
Search is cross-document and very fast compared to the Find function
— both factors are related to the mechanics of the Search function:
the PDF producer uses Acrobat Catalog in advance to prepare a
‘full-text search index’, listing all words in the document
collection being indexed. This ‘index’ (.pdx file pointing to a
folder structure with index-specific files) provides the Search
function with pointers to all occurrences of different words
(including text in vector graphics, if it is retained as text). When
the user searches for a word, it is the pre-prepared index that is
being searched, and not the document itself. When a word is found,
pointers to the locations in documents within the collection are displayed.
This means that the user in not searching within the current document, and
can search for a word without any documents being open.
With the Search function, the user must first select/activate the
index [shown above] to be used (Edit > Search > Select Indexes, or the Indexes
button in the Search dialog box). The PDF producer should assist, whenever
possible, by associating the index with PDFs in the document collection —
either with PDFs that are considered main entry points, or with all PDFs.
This way, end-users will automatically have the index activated without
having to select it manually. An exception to this is when the same PDFs are
distributed individually or placed on a site; Acrobat will display an error
message if the attached index is not present. (Acrobat Search requires the
index and all PDFs to be stored on a local or network drive, maintaining the
relative path present when the index was created. Search won’t work if the
index or PDF files are stored in a web site; there are, however, third-party
products that support PDF searches on the web).
The Search function typically searches a group of documents; when it
displays the search results, this is comprised of a list of all
matching PDF files, each shown by its title and a score. Clicking any
of the titles takes you to the first page with corresponding hits in
that file, highlighted. Clicking the Previous Highlight or Next
Highlight buttons takes you to the previous or next occurrence,
moving transparently to the next or previous file in the list of
results. The search results can be narrowed down by searching within
the Search Results, rather than searching the entire collection again
(hold down the Control or Option keys, and the Search button changes
If there is only one hit, Acrobat takes you directly to the location
in which hits are found — highlighting the matching words (without
displaying the Search Results box with document titles).
Having a meaningful PDF document title is essential, as the file name
— displayed instead of a title — is not descriptive or ‘friendly’
enough. It is also a good idea to set the opening mode of all files
to show the title in the title bar to maintain orientation as to the
item currently being viewed, so that it will be of use even if the
document is opened at the middle of file, as can often happen when
clicking the Next/Previous Highlight buttons (or, for that matter,
when following cross-file links or bookmarks). In Acrobat 5, select
File > Document Properties > Open Options, ‘Display Document Title’;
when the PDF is displayed with previous versions of Acrobat, select
‘Resize Window to Initial Page’ for a similar effect.
To take advantage of the Search function and display the list of
results so that the specific section of interest can be selected
directly, it is essential that the PDF document or document
collection is constructed as a set of independent chapters, each
being a separate PDF, and not as book/s converted to single-file
PDFs. Each PDF should have its own unique Title, Subject, Author and
Keywords fields — chapter-specific — applied consistently throughout the document
collection; these also help to pinpoint subjects of interest.
When searching a single-file book with the Search function, the
reader has to click the Next Highlight button continuously, with no
clues as to the location/context (similar to the situation when using
Find), meaning that readers back in the in the digital haystack.
When the same source material is split into multiple files and the
Search function is used, the list of results indicates the probable
sections, so that the reader can decide, based on the title, whether
to click the document. Having separate chapters also means that it is
possible to open multiple windows if necessary, each with its own
title displayed in the title bar. (Splitting a book to separate
chapters should not compromise navigation — this is possible through
the use of cross-file links and bookmarks.)
It is recommended to provide a meaningful title for the index, and
also include a brief description (including information as to options
enabled or disabled for the index).
Even when all groundwork for powerful and efficient searches is
there, readers can be helped in various ways:
- First and foremost, ‘Reader with Search’ should be indicated as a
required version (free download; the Search function is not available
in the somewhat smaller-size Reader).
- A Search button or bookmark can make the function more visible to
readers who are not aware of the difference between Find and Search.
- A brief ‘How To’ section, explaining how to use the Search
function, with screen captures showing sample queries related to the
topics in the document collection. Such a section — which may be only
3 or 4 pages long — is not meant to replace the corresponding items
in Acrobat online help, but rather to provide a quick guide to
getting results, in a way that is optimized for the specific document
collection. Such a ‘How To’ section could be bookmarked under the
- If applicable, add a ‘Common Queries’ bookmark, under which queries
likely to be used more often by readers are listed. Clicking each of
these items runs the pre-defined query and results are instantly
- The index should be activated automatically when opening the PDF. For advanced applications where multiple indexes are used
concurrently, it is possible to deselect active indexes as well.
Searching for specific information does not exclude other
access/navigation mechanisms, including bookmarks and links in items
such as a table of contents or a standard index; these complement one
another. Whereas the table of contents and index lists items directly
so that they can be selected, one has to know precisely what to look
for when using Find or Search.
PDFs in Acrobat 5 CD
Large Single-File PDFs
The major shortcoming of the Acrobat 5 PDFs, in my opinion, is the
inefficient use of the Search function. Acrobat Help (page 222)
rightly advises: ‘Consider creating a separate PDF file for each chapter or
section of a document. When you separate a document into parts and then
search it, search performance is optimized.’ However, all PDFs in the Acrobat
5 CD were constructed as a single PDF for an entire book. This applies to the
Acrobat Help itself, but also to the PDF Reference (696 pages) and even to
the gigantic Acrobat Core API Reference (2755 pages). When searching for
‘event’, for example, we get 16 books listed, with no clues as to specific
sections within these books where items are located. (It is possible to formulate
the search query for a better focus and fewer items listed, but the end
result is still entries that show the entire book.)
The Core API Reference demonstrates another potential problem, where
Acrobat Catalog splits very large PDFs to two or more parts. In the
Search Results, we see two entries which relate to the same PDF:
‘Acrobat Core API Reference’ and ‘Acrobat Core API Reference: Pages
2389 to 2755.’ While it may be possible to minimize this separation
by modifying Catalog preferences, it is best to avoid having such
large PDFs in the first place.
Text Representation Problems
Text in PDFs in the Acrobat 5.0 CD is generally ‘well-behaved’ — no
major anomalies are found.
In a few documents, spaces are missing in the ‘internal
representation’. As an example, inspect the Contents page in the
Acrobat Development Overview (DevelopmentOverview.pdf in the
Getting_Started folder in the SDK documentation). When trying to
locate the phrase ‘This Document’, which appears three times in the
top area of the page, you will not succeed. Select the text with the
Text Select Tool, copy and paste it to a text editor; you will then
be able to see that spaces are missing in different locations:
- HowThisDocument IsOrganized
Trying to find ‘ThisDocument’ will succeed in locating these
instances. A similar problem can be seen in the ‘Acrobat Developer
Extra spaces added in random locations within words are actually a more
common problem in PDFs, but in the case of the Acrobat 5 PDFs this was not
The Acrobat Help file (Help > Acrobat Help) demonstrates the problem
associated with hyphenation. The document uses moderate hyphenation,
where only longer words are hyphenated, with 5 or more characters
left on either side. These hyphens — such as in ‘accessi-bility’,
‘appli-cation’ — cause text to be interpreted differently. Searching
for plain ‘accessibility’ and ‘application’ will not locate the
hyphenated versions, but ‘accessi bility’ and ‘appli cation’ (with a
hyphen or spaces in the hyphen’s location) will succeed.
The opposite problem — of a hyphen discarded at end of line — is seen
Trying to find ‘client-side’ (typing either ‘client-side’ or ‘client
side’), we get one match. But is it the only instance? No. Using Find
with ‘clientside’ we locate another instance where ‘client-side’ is
split between lines at the ‘natural’ hyphen.
The Acrobat Distiller Parameters (DistillerParameters.pdf in
PDF_Creation_APIs) demonstrates the impact of having information
arranged in tabular form, with multi-line items. Acrobat has no idea
of the presence of table columns, which significantly reduces
retrieval of phrases split between lines. when searching for the phrase
‘sampled images’, several instances are located, but not the one in page 37.
In Batch Sequences (BatchSequences.pdf), the title in the first page
was converted to a bitmap — making it impossible to locate; a similar
problem is seen in ADBC.pdf. This problem, where larger-size
characters are transformed to bitmaps, is related to the PostScript
driver being used.
To see potential problems with products that support advanced
typography features, such as ligatures, small caps and old-style
numerals, see the Adobe OpenType User Guide, authored with
Adobe InDesign and exported directly to PDF:
- ‘2002’ is present in the first page below the title — but cannot be
located as since old-style figures are used.
- The SFNT acronym present in the first paragraph in page 2 cannot be
located, as it uses small-caps.
- ‘Microsoft’ is present 5 times in this document — but none of the
instances can be located due to the use of ligatures (ft in this
case). Even the word ‘This’ in the opening paragraph in page 2 (line
before last) cannot be located due to the use of ligatures. The more
common fi, fl, ffi ligatures are searchable in the case of this
document, but this is not the case in other documents using these
ligatures (this depends on the applications used to author/create the
While these OpenType features result in a superior typography, they
should be avoided in online documents, until Acrobat Find and Search
functions are enhanced to support the additional characters.
As an example for a PDF with text that is internally deformed, see the Adobe InDesign Programming Guide. It includes numerous code
fragments (see pages 419 and onwards) set in a monospace font, and the same
font is used in regular text to indicate function names or related items. All
of these are not searchable. Copy and paste the text and you’ll see why:
‘matrix passed’ is understood internally as ‘2#___A’.#%%_&”. With this type
of document, users could have happily used the copy and paste function to
reduce typing time/errors when studying or implementing the techniques
discussed, but results in this case are of no value.