From Adam Engst’s “Slim down your PDFs” (Macworld: 5 November 2008):
Though few people realize this, you can reduce the size of PDF files using the Leopard version of Preview. To shrink a PDF file, open it in Preview, choose Save As from the File menu, and, in the Save dialog box, choose Reduce File Size from the Quartz Filter pop-up menu. If when you compare the compressed PDF with your original, the images are too fuzzy for your needs (the default settings are pretty severe), you can make your own Quartz filter with different settings.
To do this, launch ColorSync Utility (in Applications/Utilities), choose New Utility Window from the File menu if none is showing, and click on Filters in the toolbar. Click on the arrow to the right of Reduce File Size, choose Duplicate Filter from the drop-down menu, and name your new filter. Enter different values for Image Sampling and Image Compression, switch back to your original PDF in Preview, and save a PDF with your new filter in place of Reduce File Size. With some trial and error, you should be able to arrive at a compromise that satisfies you.