In a few steps, your PDF file size can be changed: Go to the Preview app and open the PDF you want to compress. In the Preview app on your Mac, open the PDF that you want to compress. Click “Reduce file size.”. Navigate to the Acrobat online PDF compressor. Click the Quartz Filter pop-up menu, then choose Reduce File Size. Find the file size listed in the “Document Properties Description” tab. The filter provided in You can also go directly to the file itself using the files app on your device.