Create a custom PDF quartz filter

1. Open Color Sync utility in /Applications/Utilities

2. Go to Filter 

3. Select the right triangle of Reduce File Size and choose Duplicate 

4. Rename the duplicated filter and adjust the parameters as needed.

5. The duplicated filter will be saved in ~/Libraray/Filters. Move the file *.qfilter to /Library/Filters so that Preview can use it.