Oh, my favourite format during my undergraduate time! Most books in mathematics and physics (some old and niche) were available in the "Russian library".
At the same time, I haven't yet seen DjVu used in a legit way.
Licensing concerns resulted in DjVu being originally preferred over PDF by archive.org and WMF projects like Wikipedia. With baseline PDF now being unencumbered and the widespread existence of FOSS readers, PDF is both the de jure and de facto standard across even those sites.
I'm usign DJVU files every day. It's just a great format. I have a lot of archived documents which are much faster to use and require much less space than equivalent PDF documents.