The two biggest things to look at when you want to reduce the size of PDF files are removing objects and downsampling/shrinking the images it contains.

A handy trick for reducing PDF file size is to re-create or ‘re-fry’ your PDF. By creating the PDF again you can strip out plenty of unwanted objects, remove tags, and compress images further. You can do this with any tool that supports ‘print to PDF’ functionality. For this trick I’ll use the free free PrimoPDF print driver.
Open the PDF file
Open the Print dialog
Select PrimoPDF from the list of printers
On the PrimoPDF dialog, click eBooks. (Or to manually control the level of downsampling, choose Custom.)
Click OK.

Shrink all images in PDF files

For PDF files that contain many images, downsampling all of them can make a noticeable difference.
Open the PDF in your PDF editor
In Nitro PDF Professional, go to Nitro PDF Button > Prepare > Optimize Document. In Acrobat 8, go to Advanced > PDF Optimizer.
In Nitro PDF Professional, use the slider to select the level of downsampling. In Acrobat 8, in the Images tab, specify the level of downsampling.