Compressing a PDF on a Mac

macOS ships a PDF compressor, which is more than Windows does. It lives in Preview, it is one menu item, and it has exactly one setting: on. Whether that is enough depends on what is in your PDF and what number you need to get under.

The built-in route: Preview

Apple's own steps, from the Preview user guide:

1. Open the PDF in Preview.
2. Choose File, then Export. Not Save As, and not Export as PDF, which is a different item that does no compression.
3. In the Quartz Filter menu, choose Reduce File Size.
4. Click Save.

Apple's own caveat, on the same page: "When compressed, the PDF may be of lower quality than the original. The resulting file size and image quality may vary depending on the options you select."

That is accurate, and the reason is what the filter actually does.

What Reduce File Size actually does

A Quartz filter is a small description of image transformations, and Reduce File Size is one shipped with the system. It downsamples every image in the document to a fixed, low resolution and re-encodes it as JPEG at a fixed, low quality. The numbers are not exposed in the interface and have not been documented by Apple; what you can observe is the result.

On a scanned document, a 300 DPI page becomes something closer to 100 DPI and visibly fuzzy. Body text in a scan stays readable; small print and handwriting may not.

On a document with photographs, the photographs become small and blotchy, and a 20MB brochure may come out at 900KB looking like a 2009 web page.

On a document that is mostly real text with vector graphics, the filter does almost nothing, because there are no images to downsample. The text and the vector drawing were never the problem.

Document	Before	After Reduce File Size	What changed
12-page scanned contract	14MB	1.6MB	Every page softer; signatures still readable
30-page report with 8 photos	9MB	1.1MB	Photos blotchy, text untouched
40-page text-only thesis with embedded fonts	2.1MB	2.0MB	Essentially nothing
4-page brochure, large images	22MB	700KB	Images visibly degraded

The last row is the trap. The filter has no idea whether you needed 5MB or 500KB, so it applies the same heavy hand either way. If the limit was 5MB, you gave up most of the image quality for nothing.

What it cannot do

It cannot be told a size. There is no target. You get whatever the fixed settings produce, and if that is still over the limit the filter has nothing more to offer. If it is far under, it has thrown away quality you were allowed to keep.

It cannot try lossless first. Many PDFs are large for reasons that have nothing to do with image resolution: uncompressed object streams, embedded fonts duplicated per page, leftover editing history. Those can be removed with no quality cost at all, and the lossless pass is worth running before any rasterising. Preview does not have one.

It cannot tell you what it did. Nothing in the save dialog reports the resolution the images were taken to. You find out by zooming in afterwards.

Making your own filter, if you want control

ColorSync Utility, in Applications, Utilities, lets you duplicate the Reduce File Size filter and change its numbers: image resolution, JPEG quality, and a minimum image size below which it leaves pictures alone. Once saved, the custom filter appears in Preview's Quartz Filter menu.

That gives you a dial, but still no target: you set a resolution, export, check the size, and try again by hand. For one document it is fine. For the fifth document this month, it is the loop a tool should be running for you.

A quicker route that measures

The reason a fixed filter cannot land on a number is that two PDFs with the same page count and the same settings come out at very different sizes, because one has a photo on every page and the other has three. The only way to hit a limit is to encode, weigh the result, and adjust.

Compress a PDF to 1MB, to 2MB or to 5MB does that in Safari or Chrome on the Mac, without uploading the document. It tries the lossless rewrite first and measures it; a text-heavy PDF often fits right there with nothing lost. Only if that is not enough does it rasterise, and then it bisects the render scale against the real byte count, so a 5MB limit gets 5MB worth of quality rather than the 700KB a fixed filter would hand back.

The order to do things, on a Mac

1. Find out why it is big. Get Info shows the size; the number of pages and whether the text is selectable tells you most of the rest. The four reasons a PDF is large each want a different fix.
2. If it is a scan, the only thing to decide is the resolution, and Reduce File Size will pick one for you whether you like it or not. A bisection against your actual limit picks a better one.
3. If it is a real document with images, try a lossless pass first.
4. If the limit is loose, say under 5MB for a mail attachment, Preview's filter will probably do, and the quality cost may not matter.
5. If the limit is tight, or quality matters, use something that takes a target.

FAQ

How do I reduce the size of a PDF on a Mac?

Open it in Preview, choose File, Export, pick Reduce File Size from the Quartz Filter menu and save. It downsamples every image to a fixed low resolution and quality. It has no target and no quality control, so if the result is still too large or looks worse than you need, a tool that accepts a byte limit and measures its own output is the next step.

Why does Preview's Reduce File Size make the PDF blurry?

Because the filter downsamples all images to a fixed resolution and re-encodes them as low-quality JPEG regardless of what the document needed. It cannot tell whether your limit was 5MB or 500KB, so it applies the same heavy setting either way. ColorSync Utility lets you duplicate the filter with gentler numbers, but you still have to check the resulting size by hand.

Can I compress a PDF on a Mac without losing quality?

Often, if the document is text and vector graphics rather than scans. A lossless rewrite packs the internal object streams and strips duplicated fonts and editing residue, which can halve a file with no visible change. Preview does not offer that pass; it only has the lossy image filter. A compressor that tries lossless first and measures the result will tell you whether it was enough.

Is there a free PDF compressor for Mac that lets me pick a size?

Preview is free but takes no target. A browser-based compressor that runs locally can take one: it tries a lossless pass, then bisects the image quality against the real file size until it finds the best result under your number. The document stays on the Mac throughout, which for a contract or a medical record is the other thing worth caring about.