Zathura is a free, plugin-based document viewer. Plugins are available for PDF (via poppler or MuPDF), PostScript, DjVu, and EPUB. It was written to be lightweight and controlled with vi-like keybindings.
USE flags for app-text/zathura A highly customizable and functional document viewer
USE flags for app-text/zathura-meta Meta package for app-text/zathura plugins
emerge --ask app-text/zathura
emerge --ask app-text/zathura-meta
IF you use xdg-open, you might like to set it to be your default PDF application:
xdg-mime default zathura.desktop application/pdf