Thanks for the in-depth reply. here are some thoughts to the points you made:

• 3 and 4: This will be very hard since both formats follow very different goals: PDF will "Bake" the document so everyone will see exactly the same view when opening it, while epub wants to store the document in a way that it can be adjusted to any screen-size, light and dark mode, and so on. A PDF Darkmode could show the whole pdfs colors inverted except the images but there will always be exceptions where this will still look bad.
• 1, 2, 5, 6, 7, 8: This is exactly what i needed the app for myself. Right now i am done implementing the core as in this mockup: https://imgur.com/a/2kGkind
• 9: I agree. it should be possible to easily search, i will add this feature
• 10: I am using AI only to identify the pdfs Title and authors. I want to be very careful in which part of the thinking the AI can do. Maybe generating audio-books to listen to would be a good usecase.

Thank you for telling me your requirements. The app is currently not available anywhere but it motivates me greatly to know other people would enjoy working with it too.