PDF viewers
Firefox
Firefox browser can open PDF files.
MuPDF
A lightweight PDF viewer. Install package mupdf-x11. When the file manager asks for a program to open the PDF file, navigate to /usr/bin/mupdf. Handling happens mainly by keyboard, see e.g. debian manpage.
Xpdf
An X11 PDF viewer (website). Looks a bit old-school (screenshot).
zathura
A minimalistic, vim-like document viewer. Handling by keyboard, see manpage.
CorePDF
Part of CoreApps collection. Depends on qt5-qtbase.
Atril
MATE document viewer. Depends on file manager caja, itself depending on mate-desktop.
Evince
GNOME document viewer. Depends on gnome-desktop, these are just the core libraries (not the full desktop).
Okular
Document viewer of KDE. Depends among others on kwindowsystem and qt5-qtbase.
qpdfview
Lightweight PDF viewer for Qt. Not available on Alpine Linux and postmarketOS. For compliling: source on Launchpad.
Pidif
Lightweight PDF viewer using GTK4, built especially for small touch screens. Available on Alpine Linux and postmarketOS as pidif.