I am using Librera Reader. Make sure to install the F-Droid version (Librera FD) without Google Play services.

It is primarily an ebook reader that supports a variety of formats, but it is also an excellent PDF viewer. Significantly more feature-rich than any other FOSS PDF viewer for Android that I have found.

https://github.com/GrapheneOS/PdfViewer I'm using GrapehneOS and the integrated PDF viewer. Can't say of it works flawlessly on other Android OSes, but you could give it a try!

MuPDF Viewer works fine for me. Not very feature rich tho.

Mupdf: lightweight, works great.

MuPDF mini or Sav PDF viewer

MuPDF comes by default on CalyxOS, and is what I use.

I recommend the GrapheneOS Secure PDF Viewer. Unfortunately it's not on F-Droid, but you can use Obtanium to pull the apk from their GitHub: https://github.com/GrapheneOS/PdfViewer

MuPDF is another option, it even is available on F-Droid.

Mupdf or fossify file manager has a pdf reader built in

I use MuPDF but there is also the GrapheneOS PDF viewer available on their GitHub

https://github.com/GrapheneOS/PdfViewer

You can install it using Obtainium or jus grab it from the GitHub page.

Firefox Mobile can open PDFs fine - https://lemmy.blahaj.zone/pictrs/image/1c51814a-c761-4325-baa1-346e8ae2e7fb.png

I normally use muPDF

KOreader is great, but use muPDF if you don't want to learn