pdf grayscale

gs -sOutputFile=output.pdf -sDEVICE=pdfwrite -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dCompatibilityLevel=1.4 input.pdf < /dev/null


Source: http://newsgroups.derkeiler.com/Archive/Comp/comp.text.pdf/2009-06/msg00046.html