Apple’s built-in Preview app has long had a way that you can reduce the size of a PDF by recompressing and downsampling the images in the PDF.
Simply open a PDF in Preview and go to File > Export and then for Quartz Filter select Reduce File Size.
Whilst this generally provides very good savings in terms of file size, it is at the expense of the image quality in the PDFs.
Digging a bit deeper into how this is configured, it turns out that the default Quartz Filters are saved in /System/Library/Filters. Knowing that just about everything inside of /System is now off-limits, I created a new folder in /Library called Filters and put some customised versions of the filters in there instead.
After relaunching Preview, my new filters were there to be selected.
Click to download a copy of my quartz filters. It’s pretty self explanatory what they do from their name.