You can reduce the size of a Portable Document Format file on a Mac by using the built-in Preview program's Reduce File Size function. You can also use third-party tools on a Mac to reduce PDF size.
Reduce PDF Size With Preview
Preview, the OS X utility, provides a mode to reduce a PDF's file size.
Click the Applications menu on your Mac's Dock and then click the Preview icon.
When Preview loads, click the File menu and choose Open.
In the Open dialog box, choose the PDF file you want to reduce in size.
Click the File menu and, within the menu, select Export.
Type a name for the new, reduced-in-size version of the file.
Choose a file name different from the original file if you want to keep the original version.
In the dialog box, click the Quartz Filter menu and select Reduce File Size.
Click the Save button.
Reduce PDF Size With Other Tools
You can use other tools on a Mac to reduce PDF file size as well.
PDF Squeezer, available from the Mac App Store, can batch-reduce multiple PDF files at once. It shows you the difference in size between the original file and the newly reduced file.
Upload a PDF to the website PDFCompress and download a compressed version. It works from Windows machines as well as Macs and accepts files from cloud services, which may save you a step if the PDF file isn't on your Mac.
If you have Adobe Acrobat Pro, use its PDF Optimizer feature to adjust the size of PDF files, changing the settings based on how you plan to use the file.