Based on our record, Alfa eBooks Manager should be more popular than Sumatra PDF. It has been mentiond 2 times since March 2021. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
Alfa eBooks Manager (site seems to be down at the moment; more bloated than Calibre, but at least it let you keep your folder structure). Source: over 2 years ago
Not FOSS but the best ebook manager I've found is Alfa. https://alfaebooks.com/. Source: over 2 years ago
You can use your browser for both, there are extensions(FireFox - PDF & EPUB/Chrome - PDF& EPUB) for that. In case, you want you use a separate software for them, I suggest SumatraPDF--it handles both PDFs and .EPUB files. Source: over 1 year ago
calibre - Ebook manager, viewer & converter
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
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Delicious Library - Delicious Library 3: Catalog your books, movies, music, software, toys, tools, electronics, & video games. Archive your life.
Okular - Okular is a universal document viewer based developed by KDE.