Make a Better Reduce File Size Filter for Preview (ColorSync Utility)
Copy Apple’s Reduce File Size filter, rename it, and change three numbers: DPI, JPEG quality and the pixel ceiling. A Share filter at 110 DPI and quality 0.60 cut our test PDF from 3.16 MB to 0.89 MB. One setting, a DPI at or above the image’s own, made it 33 MB.
Which settings make a better Reduce File Size filter?
Preview’s built-in filter is fixed at JPEG quality 0.70, 144 DPI and a 2,400 pixel ceiling. Why it works that way, and how to edit the file, is in our Preview Reduce File Size guide. This page skips that and gives you three filters worth keeping, with measured results.
We built each from a copy of the stock file, changing only these values, and ran them on one PDF: three pages, each a 2,400 pixel photograph placed at 200 pixels per inch. Original size: 3,163,546 bytes.
| Filter | DPI | Quality | Max px | Result (bytes) | Image on page 1 |
|---|---|---|---|---|---|
| Original | 200 | n/a | n/a | 3,163,546 | 2400 px, JPEG |
| Share | 110 | 0.60 | 1600 | 894,564 | 1320 px, JPEG |
| Apple stock | 144 | 0.70 | 2400 | 2,151,046 | 1728 px, JPEG |
| Balanced | 150 | 0.75 | 2400 | 2,568,864 | 1800 px, JPEG |
| 150 DPI, high quality | 150 | 0.90 | 2400 | 2,918,706 | 1800 px, JPEG |
| Archive | 220 | 0.90 | 4000 | 33,362,129 | 2400 px, lossless |
The size tracks pixel count far more than quality. Going from 110 to 150 DPI roughly tripled the result; going from quality 0.75 to 0.90 at 150 DPI added about 14%.
Why did my custom Quartz filter make the PDF bigger?
Because of the last row. The Archive filter asked for 220 DPI on images that were placed at 200. Quartz had nothing to downsample, so it did not re-encode them as JPEG at all. We confirmed with pdfimages -list: the images changed from jpeg to image(uncompressed, flate-wrapped) and the file grew tenfold. A filter at exactly 200 DPI did the same thing, to the byte.
The rule from this test: keep the filter’s DPI below the DPI of your images. You can check the placed resolution for any PDF with:
pdfimages -list file.pdf | headThe x-ppi column is the number to stay under. (pdfimages comes with Poppler from Homebrew.) We saw this on one fixture and one macOS build, so test your own file before trusting a new filter on a batch. The effect we hit is the opposite of what the Archive name suggests: if you need to keep full quality, do not run a filter at all.
How do I create the filter in ColorSync Utility?
- Open ColorSync Utility (Applications > Utilities).
- Click the Filters tab. Apple’s eight stock filters are listed.
- Duplicate Reduce File Size, rename it (for example “Share”), and edit the image settings: the sampling resolution, the JPEG quality and the maximum pixel size.
Menu labels differ between macOS releases, so if the editor looks different from this, use the file route instead. It always works and takes one line:
mkdir -p ~/Library/Filters
cp "/System/Library/Filters/Reduce File Size.qfilter" ~/Library/Filters/Share.qfilter
open -e ~/Library/Filters/Share.qfilterChange the Name string, then the numbers: ImageResolution, Compression Quality and ImageSizeMax. Run plutil -lint on it to catch XML mistakes.
Where does the custom filter show up?
In Preview, choose File > Export… and open the Quartz Filter pop-up; saved filters appear next to Apple’s. Filters in ~/Library/Filters/ are also available from the PDF menu in any app’s Print dialog (Save as PDF) when the filter’s Printing domain is on, which it is in a copy of the stock file. That makes it the fastest way to produce a small PDF straight from Pages, Safari or Keynote. For huge Pages exports, see how to compress a PDF on Mac.
Two limits: a filter only recompresses images, so a text-only PDF will not shrink, and it applies one setting to every image in the file. Mixed documents, a logo and a full-page scan, get the same treatment.
When is Smol the wrong tool for this?
For occasional PDFs, a Share filter is enough. It is free, built into macOS, and nothing leaves your Mac. Writing your own is also the right call if you want exact control of DPI and JPEG quality.
Smol is different in what you tune. Its PDF presets (Tiny, Small, Medium, Large, Original) are pixel caps, not DPI targets, so you pick a size rather than compute one. On the same fixture the Medium preset gave 146,388 bytes, from synthetic-noise photographs that compress easily, so do not expect that ratio on your own files. Smol earns its $29 (once, one Mac) on batches and mixed folders, and from the Finder right-click, where a filter needs Preview open per file. See what each preset does, the best PDF compressor for Mac roundup, or pricing.
How we checked this
Built five filters from a copy of Apple’s Reduce File Size.qfilter and applied them to one 3-page, 3,163,546-byte photo PDF on macOS 27.0.1 through PDFDocument.write. Read sizes with stat and image encoding with pdfimages. Facts last checked . We make Smol, so treat our claims about it with the same care as any vendor’s, and see our editorial policy.
Quartz filters: common questions
Can I add my own Quartz filter to Preview?
Yes. Save a .qfilter file in ~/Library/Filters (or create one in ColorSync Utility). It then appears in the Quartz Filter pop-up of Preview’s Export dialog, next to Apple’s stock filters. Give it a unique Name so you can tell it apart from Reduce File Size.
What DPI should a Quartz filter use to reduce PDF size?
Pick a value below your images’ own resolution, or the filter will not downsample them. We measured 110 DPI as a good share setting and 150 as a middle option on a 200 ppi test file. Check yours with pdfimages -list and stay under the x-ppi value.
Is ColorSync Utility the same as Preview’s Reduce File Size?
They share the engine. Preview’s Reduce File Size is one Quartz filter stored in /System/Library/Filters. ColorSync Utility is where macOS lets you see and edit filters, and the ones you save in your Library folder show up in Preview.
Will a custom filter work on a scanned PDF?
Yes, and scans are where it helps most, because a scan is one large image per page. A lower DPI and quality can shrink it a lot, but a scan that is too soft stops being legible. Test at 150 DPI first and read the small print.
Why is my filtered PDF bigger than the original?
The filter probably asked for a DPI at or above the image’s own, so the images were rewritten without JPEG compression. On our test file that turned 3.2 MB into 33 MB. Lower the DPI below the source resolution and run it again.
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