Open the PDF in Preview
Click File
Click Export
Choose Quartz Filter
Select Reduce File Size
Save the new PDF
Use Adobe Acrobat’s Compress PDF tool
Use online PDF compressors
Remove unnecessary pages
Delete embedded images you do not need
Lower image resolution before creating the PDF
Save the document as a PDF again from the source app
Use Apple Automator to create a PDF compression workflow
Use a third-party PDF editor with compression options