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) a

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.