To compress a PDF file:

Open the PDF file and choose File > Save As.

Choose Reduce File Size from the Quartz Filter pop-up menu.
You can compress a PDF file in Preview.
This process compresses the images in the file. The compressed images look the same as the originals when viewed onscreen, but they may be of lower quality when printed.
Open the PDF file and choose File > Save As.
Choose Reduce File Size from the Quartz Filter pop-up menu.