PDF Flavours
Rick Borstein posts about flavours of PDF. Now that I work for Adobe, I can tell you there’s a lot more to PDF than I thought there was before starting at Adobe. 
Jim King, the architect of PDF, has some great and very readable presentations on his website. His Annotated PDF presentation is an excellent introduction to Postscript and PDF.
And Jim doesn’t even touch on XFA, the XML Forms Architecture that PDF also supports.
Did you know Acrobat 7.0 supports 3D objects in PDF files? Check out the samples on this page. You can load up a PDF containing a 3D model of a building, and walk around inside it in the Acrobat Reader.