PDF Press
How a PDF file is put together,
from the bytes on disk
to the pixels on the page.
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 every color space, blend mode, font type, standard 14 font, document structure, 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.