Use Preview
Open the PDF in Preview
Go to File > Export
Choose Quartz Filter: Reduce File Size
Save the compressed PDF
Use Adobe Acrobat
Open the PDF
Go to File > Save as Other > Reduced Size PDF
Save the optimized file
Use online PDF compressors
Upload the PDF
Download the compressed version
Reduce image resolution before creating the PDF
Remove unnecessary pages
Delete embedded fonts if possible
Flatten annotations and form fields
Export as grayscale if color is not needed
Use a lower-quality print setting when creating the PDF
Recreate the PDF from the source file with smaller images
Use Mac Automator or Shortcuts with a compression workflow