Here’s a simple method I recently discovered for shrinking the size of PDF’s on a Mac. It was able to convert a 5MB PDF down to 800k while maintaining perfectly readable quality. The key is to re-save the PDF applying a quartz filter to compress the images. Here’s the steps to make it work:

1. Open your Colorsync Utility, click on the “Filters” tab and duplicate the one called “Reduce File Size.”
   
2. You’ll want to bump up the quality a bit from the default on that preset- I found the following settings to be about right to achieve a 1/6th reduction in file size while preserving readability:
   
3. Now close out of the colorsync utility and open up one of your bloated PDFs. Choose the File > Save As option and on the save dialogue apply the quartz filter you just set up.
   

This has been helpful in shrinking down PDF’s that come out of my scanner. The document scanner I have (Canon MP830) is neat because it has an automatic document feeder and the ability to turn a big document into a PDF on my desktop by pushing a single button, but for whatever reason the lowest resolution setting still produces these massive files. I looked around and found various open source programs that would do the same thing but the quartz filter mechanism built into Mac OS X works perfectly and requires no extra software. Thanks to the good people on this forum for pointing out this handy technique.