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In truth the calibre viewer can only read EPUB files, if you try to read any other format (e.g. PDF) calibre will do an 'on-the-fly' conversion to EPUB - the results of which may or may not be satisfactory. The DJVU format is most often used for scanned documents, thus there often just a bunch images, which don't convert well to EPUB.
So you needed to install a native DJVU reader (Peter T posted a link) and make sure it opens DJVU files in Windows/File Explorer - providing that is in place then calibre will open DJVU files with that program.
BR
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