My previous startup worked with parsing PDFs, trying to apply NLP to the texts within PDFs - extracting titles, paragraphs, tables, bullet points etc. Oh my that was a nightmare. Sure we were doing difficult things, so that made us unique, but it was a slog. Working with different dimensions, pages upside down, sentences spanning across multiple pages etc etc.

I've also recently worked on a small tool called scholars.io [1] where I had to work with PDFs. I wasn't doing anything like parsing, but I just used existing PDF tools and libraries, which were much more pleasant, but still working on top of PDF is a challenge.

[1] - https://scholars.io (a tool to read & review reearch papers together with colleagues)