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Okular is a universal document viewer developed by KDE
"Okular combines the excellent functionalities with the versatility of supporting different kind of documents, like PDF, Postscript, DjVu, CHM, XPS, ePub and others"
Home page: https://okular.kde.org/
Format support matrix (I love this ... it shows what functions they support per format): https://okular.kde.org/formats.php
Reminds me a bit of "Evince" (there's even a link to that project on their page)
Moderator at portablefreeware.com
By the way, SumatraPDF is Portable and read a lot of file formats. Also Calibre reads PDF, ePub and Mobi.
And, yes, it would be great to have other program to read PDF, Postscript, DjVu, CHM, XPS, ePub and others.
RaphaelRB - Brazil