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1. PDF is mostly designed as a write-only (or render-only) format. PDF’s original purpose was as a device-independent page output language for printers, because PostScript documents at the time were specific to the targeted printer model. Interpreting a PDF for any other purpose than rendering resembles the task of disassembling machine code back into intelligible source code.

2. Many PDF documents do not conform to the PDF specification in a multitude of ways, yet Adobe Acrobat Reader still accepts them, and so PDF parsers have to implement a lot of kludgy logic in an attempt to replicate Adobe’s behavior.

3. The format has grown to be quite complex, with a lot of features added over the years. Implementing a parser even for spec-compliant PDFs is a decidedly nontrivial effort.

So PDF is a reasonably good output format for fixed-layout pages for display and especially for print, but a really bad input format.