Color to greyscale pdf

I am using the Canon ip1980 as printer and thankfully you need workarounds/tweaks to use it under GNU/Linux atleast in Debian Squeeze.Now if you want to take a printout then first you have to convert it into greyscale image and then take printout in case you are like me who is least bothered about purchasing a new color cartridge.I use following command to do that color to greyscale conversion of pdf files :

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