Another underrated feature: add Quartz Filters to a PDF file by Choosing File > Export, click the Quartz Filter pop-up menu, then choose a filter. You can create custom Quartz Filters in ColorSync Utility.app (Note: these are not the same as Quartz Composition Filters, which live in Automator workflows and Photo Booth.app).
Yes - agreed. This is one of the best ways to shrink the file size of a PDF: and if it’s not shrinking it enough create your own quartz filter with lower res/quality JPG or DPI settings.
For some inspiration: https://github.com/joshcarr/Apple-Quartz-Filters