To compress a large PDF on Mac for free, open it in Preview, choose File › Export, and apply the Reduce File Size Quartz filter — no third-party app and no uploading the file anywhere. This is ideal for anyone who needs to email or store a heavy scanned PDF but does not want to send a sensitive document to an online compressor. A 50 MB scan typically drops to 2–5 MB this way.
TL;DR
- macOS Preview has a built-in PDF compressor that runs entirely offline — nothing is uploaded.
- Use File › Export, then pick the Reduce File Size Quartz filter, and save under a new name.
- Expect roughly 5–10× smaller files; image-heavy scans shrink the most.
- The default filter compresses images aggressively, so tiny text and detailed charts can look soft.
- For a quality slider, use a dedicated editor (Acrobat, PDF Expert) or a browser-based compressor.
How do I shrink a PDF in Preview on Mac?
Preview is the default PDF viewer built into macOS, and its Export dialog includes a compression filter. To use it:
- double-click the PDF so it opens in Preview
- choose File › Export… (not "Export as PDF")
- in the save dialog, open the Quartz Filter dropdown
- select Reduce File Size
- rename the file — for example add "-compressed" — so the original stays intact
- click Save
The whole operation happens locally on your Mac, so the document never leaves the machine — the right choice for contracts, medical records, or anything confidential.
How much smaller will the PDF get?
Results depend on what the PDF contains:
| PDF type | Typical original | After Reduce File Size |
|---|---|---|
| Scanned document (image pages) | 50 MB | 2–5 MB |
| Photo-heavy brochure | 20 MB | 3–6 MB |
| Mostly text with a few images | 8 MB | 4–6 MB |
| Pure vector text PDF | 2 MB | 1.5–2 MB (little change) |
Image-heavy and scanned PDFs see the biggest savings because the filter downsamples and recompresses the embedded images. Text-only PDFs are already compact, so the filter has little to work with.
What is the quality tradeoff?
The default Reduce File Size filter compresses images aggressively and lowers the document's effective resolution. Small text and complex charts can look soft or slightly blurry afterward. Always keep the original until you have checked the compressed copy at the zoom level you actually need. If the output is unreadable, a dedicated PDF editor such as Adobe Acrobat or PDF Expert exposes a manual quality slider so you can trade file size against clarity, and a browser-based PDF compressor lets you pick a target size without installing anything.
How do I compress several PDFs at once?
For a whole folder of PDFs, open them all in one Preview window (select them in Finder and press Space or open together), then export each through the same Quartz filter. macOS keeps the last-used filter selected between exports, so after the first file the process is just Export → Save for each one. For true batch automation, you can also create a Quartz filter Automator action, but for a handful of files the manual loop is fastest.
Is compressing a PDF in Preview safe for the original?
Yes — Export creates a new file and never overwrites the source as long as you save under a different name (hence the "-compressed" tip). The original PDF stays untouched on disk, so if the compressed version looks too degraded you can simply discard it and try a higher-quality tool. Because everything runs locally in Preview, no copy of your document is sent to any server.
FAQ
Can I compress a PDF on Mac without any extra software?
Yes. macOS Preview includes a built-in Reduce File Size Quartz filter under File › Export, so you can compress PDFs for free without installing anything and without uploading the file.
Why is the compressed PDF blurry?
The default Reduce File Size filter downsamples images and lowers resolution to save space, which can make small text and detailed graphics look soft. Use a tool with an adjustable quality slider if you need sharper output.
Does compressing a PDF in Preview upload my file anywhere?
No. Preview's compression runs entirely on your Mac, so the document never leaves your computer — making it a safe choice for confidential files.
How small can a scanned PDF get?
Scanned, image-based PDFs usually shrink the most: a 50 MB scan commonly drops to about 2–5 MB with the Reduce File Size filter, since the embedded page images are recompressed.
Where to go next
If compression is part of a larger drive cleanup, free up the rest of your space by clearing hidden system storage on Mac and following what to delete first when storage is full. For everyday document work without installs, the privacy-first browser PDF tools and the PDF workflows hub cover compressing, merging, and converting — all processed locally in your browser.
For your phone, Cleanor for iPhone finds large videos, duplicate photos, and heavy caches in one on-device pass — nothing uploaded.