Shrinkpdf

quick'n'dirty:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/printer -r300 -dNOPAUSE -dQUIET -dBATCH -dDetectDuplicateImages -dCompressFonts=true -sOutputFile=/path/to/file.pdf /path/to/inputfile.pdf

elaborated:

http://www.alfredklomp.com/programming/shrinkpdf