Adjust density (e.g. 100x100) and quality to your needs.
Depending on your input jpeg compression might not be the best choice due to compression artifacts. You have the choice between BZip, Fax, Group4, JPEG, JPEG2000, Lossless, LZW, RLE or Zip as alternate compression methods (some only allow b/w images). For details see here.
I was able to achieve great compression ratios for scanned/photographed documents (depending on the settings). Depending on the document source, you might want to reduce the color depth (-depthargument).