Just for my own reference, the following procedure works quite well for turning greyish pdf scans into something printable:
pdfimages foo.pdf img
for a in *.ppm; do convert $a -level 50%,80% ${a%ppm}png ; done
This uses pdfimages (on OSX you can grab the xpdf package from
MacPorts) for extracting images from the pdf file, and then
ImageMagick convert (probably MacPorts
again) to set the white and black
levels
to 50 and 80 percent respectively. Without pdfimages, ImageMagick just
calls ghostscript to convert the pdf at a fixed resolution which is of
course useless for this purpose. Next time I might have a look at the
contrast-stretch and auto-level options which might be a bit more
robust.