Mac OS X 10.6 Snow Leopard
Mac OS X comes with the great PDF viewing and manipulating program Preview.app. Among its many capabilities, it provides a trick to reduce the file size of a PDF document without the need to waste money on programs like Adobe Acrobat or Nuance PowerPDF.
Just open your PDF document in Preview and choose “Save as …” from the File menu. (In recent versions of OS X, choose “Export …” instead of “Save as …”.) In the resulting dialog you will find a drop-down menu named “Quartz filter”. Choose “Reduce File Size” from this menu and save your thinned document wherever you like. (It is however advisable not to overwrite the original in this step because the result sometimes might not meet your quality requirements, in particular when reducing embedded raster images.)
Note: To create your own Quartz filter, giving you for example more control over the quality of the embedded images, use the “Filters” tab of the main window of the ColorSync utility.