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

I just released a pretty solid annotation system for Kavita's EPUB reader. It wasn't super challenging (although Kavita has a custom epub renderer).

Now doing that with PDF would actually make it challenging.

Annotation system (with pics): https://wiki.kavitareader.com/guides/readers/epub/#annotatio...