PDF Press
Learn how a PDF file is put together, then go to the specification for the details.
Learning
From the bytes on disk to the pixels on the page, in 14 short pages: objects, file structure, the page tree, content streams, fonts, transparency, security and tagged PDF. Take apart a byte-exact sample file, and check yourself with a few questions at the end of every page.
Reference
Tables to look things up in: all 73 content-stream operators, and everycolor space, blend mode, font type, standard 14 font, filter, annotation type, action type and structure type, with the version that added each one.
Specifications
Every version from PDF 1.0 to PDF 2.0, with links to download or order all 30 documents: references, errata, supplements and extensions. PDF 2.0 itself is free.