HOWTO: Reduce size of PDF file
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=out.pdf in.pdf
-dPDFSETTINGS:
/screen : low-resolution output, lowest output size
/ebook : medium-resolution output, medium output size
/printer OR /prepress : high-resolution with maximum output size