You might like the app I'm working on:
I've given up on the idea of using tablets for PDFs. I think the main issue is annotation.
I think you need a mouse and a keyboard.
This is why I stated working on Polar. I wanted to do my main reading on a laptop/desktop but sync my notes to a mobile app.
Right now the mobile sync is done by Anki but at some point I'll probably add a PWA or mobile app to sync directly.
A PWA is more compelling as it would work on ios + android of course but that requires more work.