I did a bunch of work creating pdfs using a low-level API, object goes here stuff.

As far as I understand it, at its core, pdf is just a stream of instructions that is continually modifying the document. You can insert a thousand objects before you start the next word in a paragraph. And this is just the most basic stuff. Anything on a page can be anywhere in the stream. I don't know if you can go back and edit previous pages, you might have a shot at least trying to understand one page at a time.

Did you know you can have embedded XML in PDFs? You can have a paper form with all the data filled in and include an XML version of that for any computer systems that would like an easier way to read it.