[tzs]

How did you compress it?

Gzipping the 52 MB PDF just shrinks it to 51 MB. You got it to 6.4 and kept it PDF.

[photoGrant]

GhostScript!

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf