Use convert for lossy compression of PDF
convert input.pdf -compress Zip output.pdf
Use ghostscript for lossless compression of PDF
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dBATCH -dQUIET -sOutputFile=output.pdf input.pdf
