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I'm not sure how meaningful the term "Maximum compression possible" is. For example, you could drop the resolution of images to 1 dpi, but would you be happy with how they look? Probably not... A lot of it depends on what you're happy with in your final PDF.

Probably the simplest method is to use Preview. Select File>Export and then choose "Reduce File Size" from the Quartz Filters menu.

One of the most effective ways I've found is using pdfToolbox:

http://www.callassoftware.com/en

...it's pricey, but if you just have a one off batch then the free trial version may do the job for you.
 
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