Personally I use SumatraPDF.
Can read both PDFs, ePubs and even the djvu format. And its really lightweight, unlike the bloated monster that is Adobe Reader.
Same - for Windows it’s by far and away the best PDF reader for me. It’s shocking how far down the bloat rabbit hole Adobe Reader has gone!
I personally use Zathura, it’s minimalistic and uses VIM bindings.
But if you want something feature-dense, with a way of organising your library, eReader integration, file converting and more, Calibre is pretty amazing (and the actual reader part of Calibre is quite nice to use in my opinion).
I used to use Foxit on Windows. I'm on Linux now, and I cannot recommend Okular enough.
I hate to say it because I am a Cinnamon/Gnome/GTK guy, but I think Okular may be the best Linux reader. I think it can do form filling too.
For normal use I use Firefox, but if I needed something different Okular is one place I would look pretty quickly. I have used it in the past for more tricky things. Well I'd probably look evince and be disappointed first, then move on to Okular.
