Procedure to get optimally-sized multipage pdf files:

• scan all pages to pnm lineart at 300 dpi (tiff also works but you must wait for xsane to convert each file)
• convert -density 300x300 image-*.pnm temp.ps
• ps2pdf -dPDFSETTINGS=/printer -sPAPERSIZE=a4 temp.ps output.pdf
• rm temp.ps

source: https://bugs.launchpad.net/ubuntu/+source/xsane/+bug/75384