PDF became an ISO standard in 2008, and while the official version can be purchased from the ISO website (and delivered on CD or as a downloadable PDF), Adobe has also published an unofficial version on their website which can be downloaded for free.
According to Inside PDF:
Adobe has an agreement with ISO that it can post the standard for free provided it isn’t the “official” ISO version. So the running headings and footings have been changed and the introductory pages are different, but the technical chapters are identical including page and section numbering. If you need an official version please pay for it at the ISO site. This is one source of income for this important standards organization.
Adobe should be commended for continuing to enable the PDF specification to be downloaded for free by all developers who develop PDF software.