ColorSync Utility on macOS (sorry, not sure of the right name in English) allows to add filters. I've used it to great success for down-/resizing/-sampling all

ColorSync Utility on macOS (sorry, not sure of the right name in English) allows to add filters. I've used it to great success for down-/resizing/-sampling all kind of PDFs which sometimes are basically full pictures on each page. Create own filter, add "picture compression", mode "JPEG", move the slider just a little right from the middle... Start ColorSync app, open PDF, select filter from lower left, click "Apply" and then either "Save" or "Save as..."