To reduce the file size of a PDF generated with OS X’s print to PDF feature, choose the Reduce File Size Quartz Filter from the Color Sync options in the print dialog. [From: Mac OS X Hints]
I make a lot of casual PDFs this way and reduced file size would be very welcome. I tried this on a client’s black and white Word file and there was no reduction in size. Then I tried it on a 352Kb OmniGraffle file which had originally produced a 100Kb PDF. This time the result was 122Kb.
Finally I took a 3.4Mb photo and opened it in Preview. The PDF without the filter was 14.2Mb and the one with was an amazing 32Kb. The quality was quite degraded though, unsurprisingly. While the photo was OK at normal size, a single zoom clearly showed that a lot of detail was gone.