The user interface for Adobe Acrobat X was a very different beast from previous versions and some features that were once easy to find ended up being lost in some obscure places.
One such feature was the PDF Optimizer. In Acrobat 9 Pro and earlier versions it was available from the Advanced menu — but in Acrobat X Pro it was seemingly nowhere to be found (not even in the new Panels section that Acrobat X introduced).
Well the search is over because in Acrobat X Pro the PDF Optimizer feature has been moved to:
File > Save As… > Optimized PDF
It’s strange that Adobe relegated this powerful feature to the Save As dialog, but now you know where it is, it shouldn’t be too hard to find in the future.