On linux to convert from PDF to DJVU you can use pdf2djvu

sudo apt-get install pdf2djvu

then type

pdf2djvu -o output.djvu imput.pdf

useful options are

--monochrome
--lossy
-d NNN (where NNN is the desired page resolution)

To convert from DJVU to PDF you can use djview4

sudo apt-get install djview4

then in menu File -> Export -> as PDF