- Go here and click the green Clone or download button, then select Download ZIP.
- Go to Downloads on your computer and rename the downloaded folder to Filters.
- Make a new Finder window (Command+n) and then hold Option while clicking Go -> Library in the menu.
- Drag the Filters folder with he files you downloaded into this new Library folder.
- Download this program to your Applications folder or Desktop.
- Now you can just all your PDF's files at once and drop them onto the application and it will compress them and save the compressed versions to your desktop.
Shrink PDF Files in OS X and Control Image Quality
This guide is using information from here and providing clearer instructions and a Automator script to compress many at once.