Open the PDF in Preview
Go to File > Export
Choose Quartz Filter: Reduce File Size
Save the new PDF
Use Adobe Acrobat’s Compress PDF tool
Use online PDF compressors
Reduce image resolution before creating the PDF
Remove unnecessary pages
Delete unused annotations and comments
Flatten forms and layers
Convert color images to grayscale
Use smaller image dimensions
Re-save the PDF from the source app with lower quality settings
Print to PDF with reduced quality settings
Optimize the PDF in Acrobat’s PDF Optimizer
Remove embedded fonts if possible
Split large PDFs into smaller files
Use Mac Automator or Shortcuts with PDF compression actions