How to get my Mac (High Sierra) to recognize custom Quartz filters I created to reduce file size?
I have had success on macOS High Sierra 10.13.4 with putting Quartz filters in a Filters folder within my user's Library folder:

/Users/my_user_name/Library/Filters/

in other words:

Macintosh HD &gt; Users &gt; my_user_name &gt; Library &gt; Filters

However, your Library folder is normally hidden. An easy way to navigate to it is to click on the Go menu in Finder and while the menu is down hold the Option key. Library will then appear in the list. Once you open it you can create the Filters folder which probably doesn't exist, and put your filters there.

For reference, the filters I am using are those that Josh Carr posted to GitHub.