The below tip applies to Mac OS X 10.5 and 10.6 (Leopard and Snow Leopard). I suspect it will also work with older OS X versions, and hopefully also with future updates. Please post comments if you have more insights into this.
As most users know, the majority (if not all) Mac OS X of applications support Print to PDF functionality. However, when producing PDF this way (ie by printing to Mac OS X's native PDF spooling function), there is no user control over the image compression settings in the resulting PDF file (nor over any other file-size-related features). This can lead to very big, full-resolution PDF files.
The below describes how you can compress PDF files on a Mac without using any third party software (like Adobe Acrobat).
A. The Basics
1. Open the PDF you would like to compress/reduce in size in Preview
2. In Preview, select "Save as..." from the File menu.
3. In the resulting dialogue window, select the Quartz Filter "Reduce File Size," then save the file.
Note: You probably want to save this file under a different name or in a different location, unless you are comfortable with overwriting the original, full-resolution file.
That is all for the basics. The result will be a (very small) PDF file, with average JPEG compression, which has been reduced to at least 50% of its original resolution and no bigger than 512x128 pixels. This is very small indeed.
B. More Advanced Options
If you would like to have a bigger selection of Quartz Filters available, thus allowing you to have a middle way between full-resolution and the above very compressed, very reduced PDF, you can download and install pre-set filters. One very good set of filters comes from Jerome Colas (original thread here), but you can of course download any other set of filters as well.
1. Download the filters. Valid filter files will have a .qfilter extension. Jerome's filters are here: http://idisk.mac.com/jcolas-Public
2. Move these filters into your User >> Library >> Filters folder. If this folder does not exist, you will have to create it.
3. Now, going back to the above basic work flow (see A), in the Quartz Filter selection drop-down when you select "Save as..." in Preview's File menu, you will notice that your newly installed filters appear.
C. Even More Advanced Options
You can also create your own, fully customises Quartz Filters using your Mac's ColorSync Utility.
1. Open ColorSync in your Application >> Utilities folder
2. Select the Filters icon in the top bar, and click on the "+" sign in the lower left corner to add a new filter. Name it as you see fit and hit Enter.
3. Once the new filter has been created, use the options from the drop-down menu (downward arrow to the right of the filter name) to add your desired compression and other filter options.
4. Like the downloaded filters as described in (B), your customised filter will appear in the Quartz Filter selection drop-down box in Preview's "Save as..." window.