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).
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 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.
