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DjVu (.djvu) is an open/free ebook and scanned document format that's considered less harmful than PDF, EPUB and alternatives. It's also a superior format with smaller filesizes while retaining similar quality. While yet not a common standard, it competes well against other formats.
Existing PDF files can be easily converted to DjVu with pdf2djvu. It is free software licensed under GNU General Public License version 2. It is common that PDF files converted to DjVu this way are ~60%-80% smaller in filesize.
At least SumatraPDF, Evince and djview4 are known to support reading .djvu files.