Hacker News Reader

I usually use Calibre, it's available for a variety of platforms: https://calibre-ebook.com/download

This is my current tool: EPUBReader, an add-on for Firefox: https://addons.mozilla.org/firefox/addon/epubreader/

You can upload to Google Play Books. https://play.google.com/books

It works fine on my iphone (iBooks - installed by default).

They asked about readers for desktop, specifically Windows. Pretty sure iBooks is not relevant.

iBooks run on the desktop as well and syncs back and forth.

It's my goto PDF and epub reader.

You could have mentioned it is Windows only. Thanks for suggesting it though.