[Quoth]

DJVU: scanned material, sometimes from PDFs, with extra compression and often less quality so people didn't have to use the Acrobat reader.
Though the only stuff I have are Russian manuals. I can't imagine conversion to other than A4 PDF images.

OCR in general, on DJVU or PDF image formats, as text layer is really ONLY for search, not for actual reading!
The biggest copier of paper to PDF in the world now is Google. They are only interested in more adverts and hits on their search pages, now poisoned with PDF Google books. They should have lost the court case. It's way beyond Fair Use and more to do with selling adverts than saving an archive of the world's books.