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Also (optionally) in Zathura, my preferred PDF view for its minimalist interface: https://pwmt.org/projects/zathura/

Did you know the mupdf backend allows you to open pdfs and epub? 99% of ebooks are either pdf epub or trivially colorlessly convert to epub. There is reasonable virtue in being able to use a single reader for all books.

Inkscape also offers an option to use Poppler as the renderer engine when importing/exporting PDF documents.

It's also used in Evince.

I guess the other (quite complete) open source options are xpdf itself, muPDF (used in Sumatra PDF), pdfium (Chromium's PDF viewer) and pdf.js. I wonder how they compare between each others. Especially muPDF and Poppler.