The fastest way to compress a PDF on Mac

The built-in Preview app on your Mac can reduce a PDF's file size without needing any extra software. Open the PDF in Preview, go to File > Export, choose PDF from the format dropdown, then click the Quartz Filter dropdown and select "Reduce File Size". This shrinks the file by lowering image quality slightly — usually enough to cut the size in half or more, depending on how many images the PDF contains.

If Preview's built-in filter doesn't compress enough, or if you need more control over quality settings, macOS also includes a command-line tool called Ghostscript that gives you finer options. Most people find Preview sufficient for everyday PDFs like receipts, forms, or documents they're emailing.

Key Takeaways

  • Preview's "Reduce File Size" filter compresses PDFs directly without installing anything, and works for most everyday documents.
  • The compression trades image quality for smaller file size, so text-heavy PDFs compress more than image-heavy ones.
  • If you compress a PDF and later need the original quality, you'll need the uncompressed version — compression removes data permanently.
  • For batch compression of many PDFs at once, Automator (built into macOS) can run the Preview compression on multiple files automatically.

Using Preview to compress a single PDF

Open the PDF you want to compress in Preview. You can do this by right-clicking the file, selecting "Open With", and choosing Preview, or by dragging the PDF onto the Preview icon in your Applications folder.

Once the PDF is open in Preview, click File in the menu bar, then select Export. A save dialog will appear. At the bottom of this dialog, you'll see a dropdown that says "Format" — make sure it's set to PDF. Below that is another dropdown labeled "Quartz Filter". Click it and select "Reduce File Size".

Choose where you want to save the compressed file and give it a name. Click Save. Preview will process the file and save the smaller version. The original file stays unchanged, so you can always go back to it if needed.

What happens when you compress a PDF

Compression works by reducing the resolution of images embedded in the PDF and removing some metadata (information about the document that doesn't show on screen). Text in the PDF stays sharp because it's stored as actual text, not as an image. This is why a PDF full of scanned pages compresses much more than a PDF made from a word processor.

The "Reduce File Size" filter is designed for general use and doesn't let you pick the exact quality level — it applies a preset compression that works well for most documents. If you compress a PDF and later realize you need the original quality, you'll need to go back to the uncompressed version, because the compression removes data permanently.

Compressing multiple PDFs at once with Automator

If you have many PDFs to compress, Automator (built into macOS) can run the compression on all of them automatically. Open Automator from your Applications > Utilities folder and create a new document. When prompted, choose "Folder Action".

Select the folder where your PDFs are stored. In the workflow editor on the right, search for "Ask for Finder Items" and drag it into the workflow. Then search for "Run Shell Script" and add that below it. In the shell script box, paste this command:

for f in "$@"; do /usr/bin/python3 -c "import sys; sys.path.insert(0, '/usr/local/lib/python3.9/site-packages'); from PyPDF2 import PdfReader, PdfWriter; reader = PdfReader('$f'); writer = PdfWriter(); [writer.add_page(reader.pages[i]) for i in range(len(reader.pages))]; open('${f%.pdf}_compressed.pdf', 'wb').write(writer.write())" 2>/dev/null || echo "Skipped: $f"; done

This approach requires some technical setup. For most people, compressing PDFs one at a time in Preview is simpler and doesn't require command-line knowledge.

When compression might not work well

Compression works best on PDFs that contain photographs or scanned documents. A PDF made from a word processor (like a document exported from Microsoft Word or Google Docs) often won't compress much because it contains mostly text, which is already stored efficiently.

If a PDF is already compressed (some PDFs are saved in compressed format to begin with), running compression again won't make it smaller. You can check the file size before and after compression to see if it made a difference. If the size barely changed, the PDF was likely already compressed.

Alternative tools if Preview doesn't meet your needs

Third-party apps like PDFtk, Coherence X, or PDF Squeezer offer more control over compression settings and batch processing. These are paid or freemium tools available on the Mac App Store or their own websites. They're worth considering if you compress PDFs regularly and want to set specific quality levels or automate the process across many files.

Some online PDF compression services exist, but they require uploading your file to a server. If your PDF contains sensitive information (financial records, medical documents, personal data), compressing locally on your Mac with Preview keeps the file on your device and avoids sending it over the internet.

Frequently Asked Questions

How much smaller will my PDF get after compression?

That depends entirely on what's in the PDF. A scanned document with many photographs might shrink to 30 to 50 percent of its original size. A PDF that's mostly text might barely shrink at all. The only way to know is to compress it and check the file size before and after.

Will compressing a PDF make the text blurry?

No. Text in PDFs stays sharp because it's stored as actual text, not as an image. Only images and photographs get lower resolution. If your PDF is a scan of a printed page, the entire page is an image, so yes, it will look slightly less sharp after compression.

Can I undo compression if I change my mind?

No. Compression removes data permanently. That's why Preview saves the compressed version as a new file — your original stays unchanged. Keep the original if you think you might need the full quality later.

Does compression work on password-protected PDFs?

Preview can open password-protected PDFs if you know the password, but compression may fail if the PDF has restrictions set by the creator. Try compressing it anyway — if it doesn't work, you'll get an error message.

Is there a way to compress a PDF without losing any quality?

Not really. Compression by definition removes some data to make the file smaller. You can choose how much quality to sacrifice (Preview doesn't give you that choice, but some third-party tools do), but you can't compress without losing something.