I'm not for clearing out djvu, but it sure is frustrating when a PDF isn't available.
It's not just about laziness preventing one from installing the more obscure ebook readers which support djvu. It's about security: I only trust PDFs when I create them myself with TeX or similar, otherwise I need to use the Chromium PDF reader to be (relatively) safe. I don't trust the readers that support Djvu to be robust enough against maliciously malformed djvu files, as I'm guessing the readers are implemented in some ancient dialect of C or C++ and I doubt they're getting much if any scrutiny in the way of security.
It's super easy to convert a DJVU file to PDF though. There's an increase in filesize but it's not the end of the world.
And since you're creating the PDF yourself seems like you can trust it? Since nothing malicious could survive the DJVU to PDF conversion since it's just "dumb" bitmap-based.
If your DjVu file contains an exploit for your DjVu decoder, even if you run it in a bombproof container, it could still conceivably inject malicious code into the resulting PDF file. That sounds far-fetched because the exploit payload would need to recognize that a PDF conversion was going on and respond by generating the PDF, but I remember when people thought exploiting buffer overflows was implausible, and this is not the same level of rocket science.
djvu is really quite a marvellous format, but I'm only able to read them on Evince (the default pdf reader that comes with Debian, Fedora, and probably a bunch of other distros). For my macbook I need to download a Djvu reader, and for my ipad, I didn't even bother trying because the experience would likely be much worse than Preview / Ibooks.
DJVU is supported by numerous book-reading applications, including (in my experience) FB Reader (FS/OSS), Pocketbook, and Onyx's Neoreader.
As a format for preserving full native scan views (large, but often strongly preferable for visually-significant works or preserving original typesetting / typography), DJVU is highly useful.
I do wish that it were more widely supported by both toolchains and readers. That will come in time, I suspect.
Calibre supports djvu on any platform. Deleting djvu books just because Microsoft and Apple don't see fit to support it by default would be a travesty.
