How to Compress a PDF on Mac Without Extra Apps

That 40 MB scanned contract won't get past the attachment limit — and you don't need an online "free PDF compressor" that uploads your contract to who-knows-where. Preview has a built-in Reduce File Size filter that does the job locally. Here's the workflow, what it actually does, and the fix when it squeezes too hard.

Compress with Preview

1. Open the PDF in Preview.
2. Choose File → Export… (not Export as PDF — you need the dialog with options).
3. Make sure Format is PDF, then set Quartz Filter to Reduce File Size.
4. Rename the file — something like contract-small.pdf — so you keep the original, and click Save.

Check the result with Cmd + I on both files. On scanned or image-heavy PDFs, expect dramatic shrinkage — often 60–90% smaller. The steps are identical on macOS 13 Ventura, 14 Sonoma, and 15 Sequoia. Always export a copy rather than overwriting: compression throws away image data, and there's no undo once the original is gone.

What the filter actually does — and when it won't help

Reduce File Size works by downsampling and recompressing the images inside the PDF; text, fonts, and vector graphics pass through untouched and stay razor sharp. That explains both outcomes you'll see:

• Scans and photo-heavy decks shrink massively — they're almost entirely image data.
• Text-only PDFs barely change. A 300 KB report exported from Pages is already about as small as it can be; if yours doesn't shrink, there was nothing to squeeze.

The trade-off is visible quality: Apple's built-in filter is aggressive, and photos can come out noticeably soft or blocky. Fine for a contract someone will skim and file; not fine for a portfolio.

The fix for over-crushed images: a custom filter

You can build a gentler version of the same filter with ColorSync Utility, which ships with macOS:

1. Open ColorSync Utility (find it with Spotlight) and click the Filters tab.
2. Find Reduce File Size, and duplicate it using the menu at the right of its row.
3. Rename the copy — say, Reduce File Size (Good) — and expand its Image Sampling and Image Compression sections.
4. Set a saner floor: a maximum resolution around 150 pixels per inch and the compression quality slider around the middle, instead of the stock filter's much harsher defaults.

Your custom filter now appears in Preview's Quartz Filter menu next time you export. Two or three variants — "email," "archive" — cover most needs, and you can tweak or delete them in ColorSync Utility any time. That's the undo: the built-in filter is never modified, so removing your copies restores the stock setup exactly.

Quick sanity checklist before you send

• Skim every page of the compressed copy — dense scans occasionally come out rougher than expected.
• Keep the original until the recipient confirms the file opens fine.
• Huge file but few images? The bloat may be embedded fonts or thousands of vector objects; Reduce File Size won't touch those, and printing to PDF re-renders the file instead — a different tool for a different problem.
• Selectable text should stay selectable. Because the filter only touches images, you can verify nothing structural changed: if you could search and copy text in the original, you still can in the compressed copy.
• If it's still too big to email, compression has done what it can — a 200-page scan has a floor. Send it as a link instead: Mail's own Mail Drop kicks in for oversized attachments, or share the file from iCloud Drive.
• Repeat exports don't help. Running the filter on an already-compressed copy mostly re-crushes the same images for little gain and more artifacts — always compress from the original.