In Lion, the *.qfilter files should be copied to the following location:
The filters will NOT show up in Preview if the files are copied to the user's folder (~/Library/Filters)
Alternatively, in Mountain Lion, since there is no /Library/Filters folder by default, you can put them in /Library/PDF Services
Thanks for the tip guys
Updated README, thanks for the feedback.
Just for whoever reads this issue in the future. Reduce File Size.qfilter is actually in /System/Library/Filters on macOS Sierra. While it is tempting to put more filters there, this can't be done since that directory is protected (sudo won't help).
Reduce File Size.qfilter
The drawback of putting the filters in /Library/PDF Services is that all filters will show up in the "PDF" pop-up menu found at the bottom of the Print dialog (where you will select "Open PDF in Preview"). For this reason I suggest keeping all these filters in a Filters sub-directory (e.g. /Library/PDF Services/Filters). This will only add one entry to the "PDF" menu (e.g. "Filters"), but the "Quartz Filter" menu will automatically visit the "Filters" sub-directory and populate itself.