Here are free options to optimize PDF documents on various operating systems:

On Mac OS X

On the Mac, it is possible to compress PDF files by following the steps below:

Open the PDF in Preview: Control-click on the PDF file, choose “Open With” and select “Preview” in the menu that appears on the screen.
Choose “Save As” in the “File” menu. Enter a name for the reduced PDF and specify a save location in the “Save As” window.
Click on the pull-down menu next to “Quartz Filter.” Select “Reduce File Size.” Click “Save” to complete the process.

On Windows

Free PDF Reducer for Windows(we have not tried it)

On Linux

You will need to have Ghostscript installed to run the following command:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

If these options do not work for you or if you’re looking for a more user friendly tool, you may also consider the advanced PDF optimizer available in PDF Studio, PDF editing software for Mac, Windows and Linux.