install

sudo apt-get install texlive-extra-utils or similar

crop

Not sure what the units are, but they seem to be a little less than a pixel. I just use trial and error. Order is left, top, right, bottom. Output is optional it will append -crop to the filename if you don't specify an output.
pdfcrop --margins '-50 -50 -50 -50' input.pdf output.pdf