We're working on EPUB. We want to do it right so it's taking a bit longer. A lot of our users are using Calibre to convert from EPUB to PDF.
We've seen a LOT of uptake among students/academics so PDF has been the main focus so far.
We also have HTML capture and are working to unify the EPUB and HTML capture functionality.
We're trying to be far far more than a reader and view polar as more of a spaced repetition platform with integrated reading so that it's insanely easy to read while learning using spaced repetition.
Anyway... if you have any questions AMA.
Since we (Polar) got our start here on HN (https://news.ycombinator.com/item?id=18219960), Kevin and I decided to give anybody who signs up 2 months free premium (valid until Jan 31, 2020). Just respond to my welcome email when you sign up :)
IIRC, recent development to polar was to make it much more friendly for viewing on mobile.
We're working on finishing reading up on mobile too.
then we're going to package it as a native mobile app.
And you get a free year of cloud storage if you make a non-trivial contribution.