Earlier quoted context omitted.
As someone who loves FoxIt reader, I'm building https://merrilin.ai to be the best damned ebook reader out there, to support PDF and epub. FoxIt's annotation system is one of the best I've experienced and I want to design one that is just as good if not better.
Building a robust annotation system for both PDF and EPUB is a tough technical challenge, but definitely needed. I've been working in a similar space and recently open-sourced it. It's a local-first 3D library for organizing and reading EPUBs and PDFs directly in the browser. It's called KoreShelf on Github
Now doing that with PDF would actually make it challenging.
Annotation system (with pics): https://wiki.kavitareader.com/guides/readers/epub/#annotatio...