Bless the heart of whoever looked at the PDF spec and said to themselves, "Nice, I'd like to writer a parser for this."

No joke. Yesterday I found out PDFs can have forms with JavaScript.

https://tcpdf.org/examples/example_014/

How does this even work?

I remember someone wrote a game in a PDF document...

I think this is it: https://github.com/osnr/horrifying-pdf-experiments