The PDF was/is a closed-source propriety file format.

If you really wanted to leave the Apple eco-system, it would most likely be possible to run macos in an emulator (kvm/qemu) on a pc and most likely the software too.

forget me not, i think on f-droid may be an option. it’s fairly easy to make data files for, and you could easily ask your favourite llm to wrap some data into the format.

I considered an accounting SaaS once. Only once though. The amount of constantly changing regulations would make it a very high maintenance project.