PDF files are usually small, but if you scan a long document at a high resolution or save a graphic-heavy presentation as a PDF, the file size can easily balloon to 20MB or 30MB. When you try to attach a file that large to an email, your email provider will often block it.
If you need to quickly shrink the file size of a PDF, you don’t need to buy expensive software like Adobe Acrobat or risk uploading your private documents to sketchy online compression websites. If you use a Mac, the built-in Preview application has a hidden compression tool.
How to Reduce PDF File Size with Preview
- Find your large PDF file in Finder.
- Double-click the file. Unless you have changed your default settings, it should open in the Apple Preview app automatically.
- Go to the Mac menu bar at the top of the screen and click File.
- Look halfway down the menu and click Export… (Do not click “Export as PDF”).
- A save dialog box will appear. First, give the file a slightly different name (like
Document_Compressed.pdf) so you don’t overwrite your original, high-quality version. - Look for the drop-down menu labeled Quartz Filter. Click it and select Reduce File Size.
- Click the Save button.
Checking the Results and Quality Warnings
Go back to Finder, right-click your new file, and select “Get Info” to check the file size. You will likely find that a 20MB file has been drastically reduced to just 1MB or 2MB.
A Warning About Quality: The “Reduce File Size” Quartz filter in macOS is highly aggressive. It heavily compresses any images inside the document and lowers the overall resolution to 72 DPI (web standard). While standard text will remain perfectly readable, photographs, complex charts, or scanned signatures might look noticeably blurry or pixelated.
Always open the compressed PDF and review it to ensure the quality is acceptable before emailing it to a colleague or client.
What if the Quality is Too Poor?
If the built-in Preview compression ruins your document, you cannot adjust its settings natively. At that point, you have two options:
- Use a reputable online compressor: Services like Adobe’s free online PDF compressor or Smallpdf allow you to choose between “High Quality” (less compression) and “Small Size” (more compression).
- Zip the file: If the recipient just needs the file intact, you can right-click the original PDF in Finder and select Compress. This creates a smaller ZIP file without reducing the image quality of the PDF inside.