PDF document sizes can vary a lot. Large PDF files can be difficult to store or share. The best way to reduce PDF file size is with Acrobat professional. But you can also use Preview to compress a PDF file. Here is how:
1. Open Preview (applications folder)
2. Select File > Open, select the PDF file, then click Open.
3. Choose File > Export, choose Reduce File Size from the Quartz Filter menu, and save the new PDF file.