Reduce File Size in Preview: What It Does and Why It Blurs

Open the PDF in Preview, choose File > Export…, open the Quartz Filter menu, pick Reduce File Size and save under a new name. Always save a copy: the filter is destructive and there is no way back.

On a photo-heavy document the result can be a tenth of the original. On a report that is mostly text, the file may come back almost the same size — or, occasionally, slightly larger.

A Quartz filter is a small recipe macOS applies while rewriting the PDF. Reduce File Size contains two instructions: downsample every image above a certain resolution, and re-encode images as JPEG at a fixed quality.

Nothing else is touched. Text stays text, fonts stay embedded, vector drawings keep their sharp edges, and the page count and layout do not change. That is why the same filter is brilliant on a folder of scans and useless on a 200-page contract.

• Blurry result? Your images were already close to the filter’s target resolution, and it compressed them anyway.
• No change? The PDF holds little image data, so there was nothing to squeeze.
• Bigger file? The images were stored in a more efficient format than JPEG, and re-encoding cost space.
• Colour shifts? Scanned colour profiles are rebuilt during export.

macOS lets you build a gentler or harsher version of the same filter, and it then appears in Preview’s menu for every document.

sips is excellent for images but cannot recompress the images inside a PDF, so it is not a way out here. Ghostscript (gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook) does the job well, but it is a third-party install, its presets are fixed steps rather than a target, and the licence makes it awkward to ship inside an app.

If you like the command line, the Automator route above gives you batch processing without extra software.

The filter’s real limit is that you cannot say what you need. Upload forms do not ask for "a bit smaller"; they ask for under 2 MB, under 5 MB, under 300 KB.

File Acture takes the number instead: set Compress by to Target size, type the limit, and the app searches resolutions from 300 down to 72 DPI and a range of JPEG qualities until the measured file size fits. In a test on an Apple M4, a 54 MB, 36-page scan landed at 4.84 MB for a 5 MB target in 1.38 seconds, and at 1.94 MB for a 2 MB target. Text and vector graphics were untouched, and a file that already fits is left alone rather than re-encoded.

It also does what Preview will not: dozens of files in one run, grayscale conversion for documents that carry no colour, and a clear message when your target is impossible instead of a quietly ruined copy. Everything runs on your Mac; nothing is uploaded. See how target size works.

The two limits people hit most: an email attachment cap and an upload form.