Hi Stefan, that sounds fantastic - it's not a massive pain at the moment, but saving to Drive would be just a useful extra feature to have, glad you're considering it!
As regards the PDF page number problem, I totally understand that having the reader analyze a document to determine page numbers would be a very complicated thing to do. I was wondering, instead, if it would be possible to basically just have a dialogue box where the user could manually tell the reader from which page number it should start counting?
So instead of having the reader try and determine which pages were which itself, the user could just manually tell it something along the lines of 'Page 1 = Page 120' and then the reader could just adjust its page counting up by the necessary amount (so start counting pages from '120' instead of from '1').
Hope that makes sense!
(On a side note, it's absolutely fantastic to see developers being so responsive to user, and it's been great seeing Paperpile develop over time - I'm really looking forward to the future of the app, it just seems to keep getting better and better!)