The best compression technique I use for making PDF's smaller without loosing quality is saving it using the Adobe PDF Preset: **PDF/X-1a:2001**. 

![ Screeshot of the "Save Adobe PDF" Panel in Illustrator CS6 ][1]

How it makes the file smaller is by defining a set of rules that forbid the use of certain PDF functions that are irrelevant for printing purposes and to enforce others that do improve its usefulness in prepress.


  [1]: https://i.sstatic.net/3NK3D.png