Reduce PDF size on a Mac

You can use the Mac Preview app to reduce the size of any PDF file. The default steps are

1. Open the PDF with the Preview app
2. Select File -> Export… (do not choose Export as PDF)
3. Select the format, which is PDF
4. Choose the Quartz filter named Reduce File Size
5. Save

It might happen that the quality of scanned images in the resulting PDF is not good enough. In this case you can create your own Quartz filter.^[1]

1. Open the ColorSync Utility, you can search for it with your Spotlight search.
2. Switch to Filters
3. Add a filter by clicking on the + symbol in the lower left corner
4. Give your new filter a name, e.g. Reduce PDF Size
5. Go to the settings of the filter by clicking on the gear symbol ⚙
6. Choose Add Image Effects Component -> Image Compression
7. Change the mode to JPEG and adjust the Quality slider to something like 40% (by default it is on 50% for the new filter).

Once you created this custom filter, you can again switch to the Preview app, choose File -> Export…, select your new filter Reduce PDF Size and Save your PDF. This should reduce the PDF size while maintaining acceptable quality.