Do something like:
for i in $(seq 0 $((pages-1))) ; do convert -density 300 document.pdf[$i] -quality 90% page-$(printf %03d $i).jpg ; done 

It's possible to have imagemagick do the whole thing in one go (it automatically
adds numbers to the output filenames):
convert -density 300 file.pdf -quality 90% page.jpg
But this eats a lot of memory and can make the system ground to a halt, and
`convert` won't generate each output file one-by-one; it will extract everything
to temporary files first.

This is a good solution for making images that can then be put together in a CBZ
/ CBR file or such, which is handy for dealing with slow-to-render PDFs
(although the resulting file is much (at least 10x) larger than the PDF).

It's also possible to put the rasterized pages back together into a PDF, which
will, like the CBZ, be much bigger, but should render faster than the original
PDF:
convert page-*.jpg rasterized.pdf

The above settings are about right for generating acceptably readable pages;
although they look high quality, we don't actually want to drop either the
density or the quality much lower.

To output monochrome files, add:
-colorspace Gray