[dbrueck]

Others have given some good answers already, but I'll add one more: PDF is all about creating a final layout, but given a final layout, there are an infinite number of inputs that could have produced it, but if you are parsing the PDF, most of the time you are trying to get back something higher level than e.g. a command to draw a single character at some X,Y location. But many PDFs in the wild were not generated with that type of semantic extraction in mind, so you have to sort of fudge things to get what you want, and that's where it becomes complex and crazy.

For example, I once had to try to parse PDF invoices generated by some legacy system, and at the bottom was a line that read something like, "Total: $32.56". But in the PDF there was an instruction to write out the string "Total:" and a separate one to write out the amount string, but there was nothing in the PDF itself that correlated the two in any way at all (they didn't appear anywhere close to either other in the page's hierarchy, they weren't at a fixed set of coordinates, etc, etc.).