FileProInfo.com is the file information database of over 15,000+ file types, software, free online apps/tools, so tools, guest blogs, videos, and images.
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Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Convertio - File Conversion in the Cloud
calibre - Ebook manager, viewer & converter
File-Extensions.org - Large computer file extension library with detailed explanation of each file type with links to download free or associated software programs.
Okular - Okular is a universal document viewer based developed by KDE.
FileInfo.com - FileInfo is the central file extensions registry and contains a database of over 10,000 file types.