I'm not very familiar with *.cbr / *.cbz, but it seems you'll have to combine two steps:
- Convert PDF to Images
- Compress them into a ZIP / RAR archive.
Regarding step 1, you could use ImageMagick's convert command. You can feed convert with a PDf comprising multiple pages, and convert will return each page as single graphics file. I've tested it with a text scanned at 400 dpi, and the following command resulted in nice single JPGEs:
$ convert -verbose -colorspace RGB -interlace none -density 400 -quality 100 yourPdfFile.pdf 00%d.jpeg
(credits regarding the -quality option: this forum entry)
As a result, you get 000.jpeg, 001.jpeg and so on. Just zip them into a .cbz file, and you're done.
You could even combine both steps by "concatenating" them:
$ convert -verbose -colorspace RGB -interlace none -density 400 -quality 100 yourPdfFile.pdf 00%d.jpg && zip -vm comic.cbz *.jpg
(make sure that there aren't any other JPEGs in your current working directory, since using the code above, zip will move all JPEGs into the cbz file)