How to Reduce PDF File Size on Mac (Preview)

A 40 MB PDF that's mostly scanned pages doesn't have to stay 40 MB. Preview — already on every Mac — can re-export it at a fraction of the size. And if the built-in filter crushes your scans too hard, a two-minute trip to ColorSync Utility gives you full quality control.

Export with the Reduce File Size filter

1. Open the PDF in Preview.
2. Choose File → Export… from the menu bar (not Export as PDF — you need the dialog with format options).
3. Make sure Format is set to PDF, then set Quartz Filter to Reduce File Size.
4. Save under a new name — keep the original until you've checked the result.

Select both files in Finder and press Cmd+I to compare sizes. On image-heavy documents the drop is dramatic; a phone-scanned contract often shrinks by 80–90%. The filter works by downsampling and recompressing the images inside the PDF — text, being vector data, stays perfectly sharp. That's also why a PDF that's already mostly text barely shrinks: there's nothing heavy to compress.

The steps are identical on macOS 13 Ventura, 14 Sonoma, and 15 Sequoia.

Two caveats worth knowing before you commit. First, the export always produces a copy — Preview never compresses a PDF in place, which is exactly what you want: if the output looks rough, delete it and try again; the original is your undo. Second, quality loss is one-way. Once you've compressed a scan and deleted the source, no tool can bring the detail back, so check any document you might need to zoom into — floor plans, receipts with small print, anything with signatures — before trashing the original.

Better quality: build a custom Quartz filter

The stock filter has one setting and it's aggressive. If your scans come out soft or blotchy, make your own filter with sensible limits:

1. Open ColorSync Utility (in Applications → Utilities).
2. Click Filters in the toolbar, find Reduce File Size, and use the menu at the right edge of its row to Duplicate Filter.
3. Expand your copy, then expand Image Sampling. Set the resolution constraint to around 150 pixels per inch — plenty for on-screen reading (use 300 if the PDF will be printed).
4. Expand Image Compression and drag the quality slider to the middle or slightly above.
5. Rename the filter something like Reduce – Readable and quit ColorSync Utility.

Your filter is saved as a .qfilter file in ~/Library/Filters, and it now appears in Preview's Quartz Filter menu in the Export dialog alongside the built-in ones. Nothing about this changes system behavior — to undo it, just delete the filter file or remove it in ColorSync Utility.

When Preview isn't the right tool

• The PDF is already small and text-only. There's nothing meaningful to compress; don't degrade it for a few KB.
• You need to hit an exact size limit (a 2 MB upload cap, say). Preview can't target a size — a dedicated compressor app can iterate quality settings for you.
• The document is sensitive. Skip online "compress my PDF" sites: uploading a contract or medical record to a random server to save disk space is a bad trade. Do it locally.
• The bloat isn't images. Some generators produce PDFs full of redundant objects. Re-saving via File → Print → PDF → Save as PDF rebuilds the file and can shrink these — but note it strips hyperlinks, so use it as a last resort.

Shrinking PDFs to free up space?

If the real goal is disk space, compressing PDFs one at a time is slow going — a handful of forgotten videos usually outweighs every PDF you own. Start with our guide to finding the largest files on your Mac and attack the top of the list first.