A simple way to reduce a pdf size using ghost script:
~$ gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=reduced.pdf input.pdf
Where reduced.pdf is the name of output file and input is the file that you want to reduce. I have no idea either time to find out what the other options do =)
UPDATE 30/11/15:
If the -dPDFSETTINGS=/screen setting is too low quality to suit your needs, replace it with -dPDFSETTINGS=/ebook for better quality, but slightly larger pdfs. Delete the setting altogether for the high quality default, which you can also explicitly call for with -dPDFSETTINGS=/prepress (source).
That’s all.