Compress a PDF on a Mac without wrecking the scans
· 6 min read
Preview can compress a PDF in one step: File, Export, then choose Reduce File Size from the Quartz Filter menu. On a text document it works fine. On scanned pages it is frequently too aggressive, and the output comes back soft, blotchy, or occasionally larger than what you started with.
The fix is to understand which part of the file is heavy before you compress anything, because a PDF made of text and a PDF made of photographs of paper are two different problems with two different answers.
Work out what is actually big
Select the file in Finder and press Command-I for Get Info. Note the size. Then open it and look at a page closely at 100% zoom.
If you can select the text with the cursor and the letters stay crisp as you zoom in, the pages are vector text. A document like that is usually already small, and if it is not, the weight is in embedded fonts, images placed on the pages, or attachments rather than in the text itself. Compression will win you very little.
If you cannot select the text, or the letters blur into gray mush as you zoom, each page is an image. This is a scan, and effectively all of the file size is image data. Everything below is about that case.
One quick sanity check while you are there: divide the size by the page count. If a page of black text on white paper is costing you several megabytes, it was captured at a resolution or a color depth the document never needed, and there is real room to work with.
The one-click route, and its problem
- Open the PDF in Preview.
- Choose File, Export.
- Set Format to PDF.
- Open the Quartz Filter menu and choose Reduce File Size.
- Give the new file a name, and save it somewhere other than on top of the original.
Then open both and compare the same page side by side at 100%.
The reason this so often disappoints is that the filter has one fixed idea of what a small PDF looks like. It downsamples images to a low resolution and re-encodes them at a low JPEG quality, with no setting to negotiate. On a slide deck exported to PDF, nobody notices. On a scanned contract with an initialed clause and a handwritten date, the result can be genuinely hard to read, and on a scan that was already efficiently compressed the re-encode can add size rather than remove it.
Keep the original until you have looked at the output. Preview’s export writes a new file, so that costs you nothing.
Make a gentler filter of your own
macOS lets you build your own version of that filter with different numbers, using ColorSync Utility (in Applications, Utilities). Open it, look for the Filters section, duplicate Reduce File Size, and give the copy a name you will recognize in Preview’s menu.
Inside the copy there are two settings that decide everything:
- Image sampling, which sets the resolution images are reduced to. This is the one that destroys scans. Raising it toward the resolution the pages were scanned at, or capping it around the resolution your recipient will actually print at, keeps the letterforms intact.
- Image compression, a quality slider for the JPEG re-encode. Moving it up trades size for the mottling that shows first in gray areas and around thin strokes.
Save it, and the filter appears in Preview’s Quartz Filter menu next to the built-in one.
Two honest caveats. Apple has moved things around in ColorSync Utility across releases, so the Filters section may not sit where it did on an older Mac. And this is trial and error: expect to export the same document three or four times before you settle on numbers you like.
Fix it before it becomes a PDF
The most effective compression happens at the scanner, and it costs nothing in quality because the data was never captured in the first place.
In Image Capture, or in Preview’s File, Import from Scanner sheet, show the details and look at three settings:
- Kind. A page of black text on white paper does not need three color channels. Grayscale is the sensible default for documents; Text or Black and White is smaller still, but it makes a hard decision about every pixel and is unkind to faint pencil, stamps and signatures.
- Resolution. Higher is not better past the point where the smallest character on the page is clearly formed. Scanning pages of ordinary body text at the highest setting your scanner offers produces files nobody can email, for no visible gain.
If the scan already exists and cannot be redone, that is fine. But if the same documents cross your desk every week, changing this once is worth more than compressing forever.
Do not compress away the text layer
A scanned PDF that has been through optical character recognition carries an invisible text layer over the page images, which is what makes it searchable and selectable. Downsampling the images does not delete that layer, so the file stays searchable even when the page looks worse. That is the good news.
The bad news is the order of operations. If you compress a raw scan hard and then run recognition on it, the recognition has less to work with and makes more mistakes. Recognize first, then compress. Make a scanned PDF searchable on a Mac covers the recognition step.
When the file is big for a reason that is not the pages
Sometimes the pages are innocent. A PDF can carry embedded file attachments, full sets of embedded fonts, layers from a design application, and thumbnail caches of pages as they used to look. None of that shows on screen and all of it counts toward the size in Get Info.
If a document is far larger than its pages can explain, that content is worth finding rather than crushing, because some of it is also a disclosure problem. Removing it shrinks the file as a side effect.
Basalt has a Compress tool with the image resolution and quality exposed rather than fixed, an Inspect tool that lists what a PDF is still carrying, and a Clean Up tool for stripping layers, bookmarks and stale thumbnails. Every tool writes a new file and leaves the original alone, and it runs on the Mac with no upload, which is the point when the scans are anything sensitive. It is a paid app after 24 hours free with every tool, no account and no card. If Preview’s filter gives you a file you are happy with, use Preview.
Questions
Why did compressing make my PDF bigger? Because the images inside it were already compressed efficiently, and the filter decoded and re-encoded them anyway, adding its own overhead. When that happens, the honest answer is that the file is already about as small as it goes without losing something you care about.
What size should I aim for before emailing? Every mail system sets its own limit, so rather than guess, get the file comfortably under ten megabytes or send it as a link from your own storage. If it will not go under, splitting the document is often more useful than compressing it further: Split a PDF into separate files on a Mac covers that.
Does compressing a PDF remove annotations or filled form fields? Preview’s export keeps them as live objects. The print route, File, Print, then Save as PDF, flattens them into the page and re-encodes everything, which sometimes shrinks a file and sometimes does the opposite.
I merged several scans and now the file is enormous. What now? Compress the individual scans first, then merge the results, so you are only looking at one page’s worth of damage at a time while you choose settings. Merge PDFs on a Mac without uploading them anywhere covers the merge itself.