1200 pages of specification, referencing about fifty other documents. Not complete, not even self-consistent, but essential.
One of O’Reilly’s Hacks series – a little book about generating, manipulating, annotating, and consuming PDF information. Lots of stuff which doesn’t need Acrobat, including free resources.
All too rare these days, a book which pulls together a whole field, from detailed chapters of explanatory prose and historical information, right down to Type 1 and Truetype font formats in detail, and a few chapters on TeX fonts, too.
Twelve years after it’s latest edition, and almost twenty since it’s first, still essential reference material.
Pretty much the only sensible source for information on CCITT fax encodings, which PDF uses for one bit-per-pixel bitmaps. The standards documents are virtually impenetrable.