Compress a PDF on Mac: Preview's filter vs a real size target

Preview and getPDF side by side

	Preview, Reduce File Size	Preview, your own Quartz filter	getPDF compressor
Cost and install	Free, built in	Free, built in	Free, in the browser
Where the file goes	Stays on the Mac	Stays on the Mac	Stays on the Mac
Picture resolution	Capped at 512 pixels a side	You set it, in ColorSync Utility	300, 150 or 110 dpi presets; a target goes from 300 down to 50
Hit a size like 500 KB	No	No, trial and error	Yes, type the target
Shows what changed	No	No	Size before and after, the dpi used, the pages that changed most
Flat graphics and logos	Not documented	Not documented	Kept lossless (64 colours or fewer)
Result not smaller	The export is saved as it came out	The export is saved as it came out	Original returned, with a sentence why

Preview facts from Apple’s Preview User Guide (“Reduce the size of a PDF”) and the filter descriptions cited below, checked on 11 October 2026; getPDF facts from the tool itself.

What Preview’s Reduce File Size actually does

Apple’s Preview User Guide gives the steps: open the PDF in Preview, choose File, then Export, choose Reduce File Size in the Quartz Filter pop-up menu, and click Save. If a file of that name exists, Preview asks before replacing it. Apple’s guide also warns that a compressed PDF may be of lower quality than the original, and that is the whole story in 1 line.

What the guide does not say is how hard the filter squeezes. Eric Meyer described it in February 2010: the built-in filter scales pictures down by at least half and to no more than 512 pixels on a side, with strong JPEG compression. OWC’s blog described the same 512-pixel cap in July 2020. You can check the filter on your own Mac: open ColorSync Utility (in Applications, then Utilities), go to Filters, and expand Reduce File Size to see its image settings.

What 512 pixels means on paper is simple arithmetic. An A4 page is 11.69 inches tall; a full-page scan capped at 512 pixels along that side ends up at about 44 dpi. That is below the strongest step of our size target (50 dpi), and it is why small print in a scan turns soft after Preview’s filter. For a photo placed small on a page, the same cap can be harmless; for a full-page document scan it is severe.

What the getPDF presets do on the same kind of file

We measure on our own test files with the same code the browser runs. Our 10-page test scan (an invented lease, 300 dpi colour, 17.6 MB) gives, as measured on 11 October 2026:

Setting	Result	Pictures at
Print preset	unchanged, 17.6 MB (already 300 dpi; the tool says so)	300 dpi
Web preset	3.2 MB, 82 % smaller	150 dpi
Email preset	1.7 MB, 90 % smaller	110 dpi
1 MB target	637 KB	72 dpi
500 KB target	366 KB	60 dpi
200 KB target	197 KB	50 dpi

Each line is a choice you make, not one the tool makes for you. Run Preview’s filter on your own file and compare the two results at 200 percent zoom on the smallest text: that is the comparison that matters for your document.

When Preview is the right answer

Use Preview when all of these are true:

• Nobody named a size. You just want “smaller”, for a file people will glance at on a screen.
• The pictures are photos, not pages of text. The 512-pixel cap hurts scanned text most; a photo that fills a quarter of a page loses less.
• You will check the result. Open the export, zoom in, and if it reads, you are done. It is quick, free and offline, and nothing else is needed.

We say this plainly because it is true: for a casual “make it lighter”, Preview is already on your Mac.

When you need a number, or control

Portals and mail servers do not accept “smaller”; they accept a number. A visa portal at 1 MB, a job site at 500 KB, an exam form at 200 KB. Preview cannot aim at a size, so you export, check, and try again. The compressor here aims for you:

1. Open the compressor above, or getpdfs.app/compress-pdf, in Safari, Chrome or Firefox.
2. Drop the PDF onto the page, or click Choose files and pick it in Finder.
3. Pick a preset (Email is the default) or type a size target in KB or MB. The ready-made pages set it for you: 1 MB, 500 KB, 200 KB, 100 KB.
4. Click Compress. With a target, the tool tries its settings from 300 dpi down and keeps the gentlest one that fits, whatever the preset, then tells you which: “Under 1 MB (637 KB) with 72 dpi pictures at quality 45, the best quality that fits”, for example. A file already under the target comes back untouched. If even the strongest step cannot reach the target, it says that too, with the size it did reach.
5. Click Download PDF. In Safari the copy goes to your Downloads folder; in Chrome a Save dialog lets you pick the folder. The copy is named after your file with -compressed on the end.

Text in the PDF is never re-encoded, so typed text stays sharp, selectable and searchable whatever you pick. A target counts 1 KB as 1,000 bytes, the stricter reading, so the file passes a portal however it counts.

Custom Quartz filters, for tinkerers

You can make Preview less harsh. In ColorSync Utility, under Filters, duplicate Reduce File Size, give the copy a name, and change its Image Sampling settings (a resolution such as 150 instead of the 512-pixel cap) and its JPEG quality. OWC’s 2020 walkthrough shows the steps, and the new filter then appears in Preview’s Quartz Filter menu.

Most people should not bother. Several walkthroughs report that a custom filter sometimes does not show up in Preview until it is moved into a system Library folder, and the location has changed between macOS versions. You also still cannot aim at a size: a filter is a fixed recipe, and you find out what it produced after the export. If you need the same recipe every week, a custom filter is a fine tool. If you need a number today, a size target is quicker.

The honest part: both are lossy

Neither route is lossless. Preview’s filter and our presets both re-encode pictures, and detail that is dropped does not come back. That has 3 consequences, whatever you use:

• Export a copy; keep the original. Preview asks before replacing a file with the same name; give the export a new name. Our tool never touches the original and names the copy -compressed.
• Check at 200 percent on the smallest text, any stamp and any signature before you send.
• A text-only PDF barely shrinks with either. Its size is text and fonts, not pictures; our tool says so and hands the original back unchanged.

There is more on what “without losing quality” can and cannot mean in Compress a PDF without losing quality, and the whole toolbox of size levers is in Make a PDF smaller.

Nothing is uploaded, from either

Preview works on the Mac, and so does the compressor here: it runs in your browser, and the PDF never leaves your machine. You can check it: compress a file once (so the engine, about 4.6 MB, is kept in the browser), turn Wi-Fi off, reload the page and compress another. It still works. On a PC, the same comparison with Windows’ own options is in Compress a PDF on Windows.