Kevin, are you familiar with SuperMemo’s incremental reading? There’s a couple ideas from there that could be great for Polar. Namely, a mode or a button that will bring up a random pdf. The user can then read for as long as their current interest holds, then randomize to the next article. It’s a great way of moving incrementally through an overwhelming number of PDFs.
If you implement your own flashcard mode, I suggest the option to review one’s highlights in a spaced repetition manner (without having to turn them into questions). There’s a service called readwise that let’s you do this with Kindle and Instapaper highlights, and I can see it being useful here. I have found this to be a powerful way to return to ideas and connect them to new ideas I’ve encountered. Unlike traditional question-based spaced repetition, the goal is not necessarily to keep ideas retrievable in your memory, but to see them mixed with other ideas, making it possible to make novel connections.
So why not just use SuperMemo? It doesn’t have support for pdf, so this would fill that whole (and be an incredible knowledge management system on its own).
Thanks for the incredible work so far.