Smallpdf [1] probably deserves a mention here. Not OSS and not self-hosted, but I‘ve used it occasionally and it has always worked really well. When I was running an agency, we inherited their first office – very cool folks.
And then there is 2.0, and all the extensions [1]
And multiply that with the number of implementations.
If the goal is to make something that "always" works, you probably need a big team to keep up with the moving field of various bugs and reimplementations
[1] https://www.loc.gov/preservation/digital/formats/fdd/fdd0000...