Compress a PDF on a Mac: Preview and quality control

Preview: export the PDF as a smaller copy

You don't need any additional program to compress a PDF on a Mac. The Preview app can export the file as a new copy using a Quartz filter, and depending on the content, the copy can turn out smaller than the original. The following steps and menu names are based on the German-language Apple guide to Preview in macOS Tahoe 26. In a different macOS version, labels and dialogs may differ. Apple's general Preview help can be switched to other macOS versions.

There's one limitation you should know about beforehand: Apple points out that image quality and final file size depend on the options you choose. So the filter can visibly degrade images. Always save the result under a new name and then compare it with the original.

1. Open the PDF in Preview.
2. In the menu bar, choose “File” > “Export”.
3. In the export dialog, open the “Quartz Filter” menu and choose “Reduce File Size”.
4. In the “Export As” field, change the file name, for example from “Zeugnisse.pdf” to “Zeugnisse-klein.pdf”.
5. Choose a location where you'll find both files again, and click “Save”.
6. If Preview asks whether an existing file should be replaced, cancel, change the name and save again.

Can I keep the original file?

Yes. Give it a new name when exporting and don't confirm any replace prompt, and the original file stays unchanged. You'll need the original anyway: if the copy is too blurry or still too large, you start every further attempt again from this unchanged file. If you want to be completely sure, also put an extra copy of the original in a different folder beforehand. A name suffix like “-klein” also prevents you from accidentally choosing the wrong version when uploading.

What quality changes are possible?

“Reduce File Size” is no guarantee of lossless compression. Whether and how much a particular PDF changes visibly can't be predicted, because Apple ties image quality and file size to the options chosen. Content stored as images in the PDF is most likely to be affected: photos, graphics, stamps and scanned signatures. In a scan, the visible page image consists of one or more images, so the text you read on it can also become blurrier. If the scan additionally has a searchable text layer from text recognition, that layer is a separate component of the file. So check separately whether search still works in the copy.

Why is my PDF blurry after the Quartz filter?

The filter saves space in the image data, and that can be visible. If a scan or a photo then looks washed out, blotchy or hard to read, the export wasn't suitable for that file. Such a copy hasn't been compressed while preserving quality, even if it meets the size limit. Discard it and keep working with the original instead of processing the blurry version again.

Custom Quartz filters with JPEG or lower resolution aren't lossless

Some guides online recommend creating a custom Quartz filter with the ColorSync Utility that saves images as JPEG at a fixed resolution such as 144 dpi. Such a filter is occasionally described as lossless. That isn't accurate: when images are re-saved as JPEG in the process, image information is lost. A resolution setting takes effect wherever an image is actually downsampled, and fine detail disappears in the process. Whether the result is good enough for your purpose depends on the file and on whether someone needs to read small print on screen or print the documents. If you try such a filter, check the result just as strictly as with the standard filter, that is, side by side with the original at the same zoom level.

How to compare the original and the copy

1. Open the original and the smaller copy side by side in Preview.
2. Set the same zoom level in both windows.
3. Look at the same spots in both files: the smallest text, footnotes, numbers in tables, stamps, signatures and photos.
4. Go through all pages, not just the first.

The guide Compare quality properly explains how to choose meaningful sections and what can actually count as lossless for a PDF.

The PDF stays too large or becomes blurry

If the first export isn't enough, it's tempting to run the copy through the filter again right away. That usually doesn't get you further, and there are better next steps.

Why doesn't compressing multiple times help?

In an older post on a Mac forum, someone describes how a job application portfolio got smaller on the first pass with a compression function from the print dialog, but further passes hardly changed anything and the file stayed above the required limit. One possible explanation lies in the content: if the images were already reduced in the first pass, a further pass with the same setting often has little room left. If lossy-compressed images are re-encoded as JPEG in the process, quality can suffer on top of that. Another attempt may still save a little, but it's more reliable to go back to the unchanged original and find out what the file consists of:

• Scanned pages: Here the size lies in the image data. If you can rescan the original document, suitable scan settings help more than an after-the-fact filter. The guide Compress a scanned PDF explains how.
• Documents exported from another program: A fresh export from the source file with different settings can provide a better basis than the finished PDF.
• Several documents in one file: If the recipient accepts several files, you can split the PDF into several files without leaving out any content. If it contains pages that aren't required according to the specifications and that nobody else needs either, you can remove them from the PDF. Both tools accept ordinary PDFs; they may reject digitally signed documents or files with form fields.
• Different method: The browser tool from the next section works differently from the Quartz filter. You can also try it with the original.

You'll find a detailed troubleshooting guide under When the PDF won't get smaller.

The Quartz filter is missing from the export dialog

• Check whether you've actually opened a PDF. Apple's guide describes the steps for PDF documents. It doesn't apply to an open image.
• Check whether PDF is set as the format in the export dialog.
• Use the “Export” command in the “File” menu. Depending on the version, there's another, similar-sounding command there for exporting as PDF. Apple describes the filter with the regular “Export”. Don't rely on the other command offering the same options.
• Compare the steps with Apple's help for your macOS version, because menus can differ between versions.

You need an exact size

With the Preview method described here, you choose a filter, but not a target size. You only see how large the copy has become after saving. If a portal specifies a fixed upper limit, Compress a PDF to a target size can help. There you set the limit and see whether the result meets it. Whether that succeeds with your file depends on its content.

Telling browser and native app apart

Besides Preview, there are other routes on a Mac that are often lumped together. They differ in where your file is processed and what happens to it in the process.

So saving via the print dialog is something different from exporting with the Quartz filter. Choosing a smaller paper size in the print dialog doesn't reliably make the saved file smaller either.

What “in the browser” means here

Our tool to compress a PDF in the browser reads the selected file in the browser, compresses it there and offers the result as a new file to download. The contents of your PDF aren't uploaded for this. The browser does, however, load the program itself from the internet, partly only during processing. So keep the connection active until you've saved the result. The guide Compress a PDF without uploading explains the difference between “without upload” and “truly offline”.

There are three methods to choose from. “Lossless optimization” improves internal storage without deliberately degrading images. “Balanced” and “Smaller file” can additionally re-save suitable images as JPEG. Image resolution isn't reduced in any method, and no page is converted into an image. You can select files up to 50 MiB. The tool doesn't accept digitally signed or encrypted PDFs.

If processing in your browser aborts or you aren't offered a result to save, first check your internet connection and reload the page. If it still doesn't work after that, do the same task with Preview, as described above.

Does PDF24 Creator work on a Mac?

No. According to the manufacturer's download page, the desktop program is only available for Windows; the manufacturer explicitly rules out Mac and Linux in its FAQ. On a Mac, use Preview or a browser-based tool. Both are independent routes with their own characteristics and not a replica of the Windows program. If you also have a Windows PC, you'll find the options there in the guide Compress a PDF on Windows.

Check the size and functions of the saved file

Before you upload or send the smaller copy, open exactly the saved file again, not the window you exported from. That's the only way to check what the recipient will actually receive.

1. Read the size of the saved file in bytes and compare it with the requirement. To do this, you can select the file under Check upload limits, for example.
2. Go through all pages: is everything complete, in the right order and nothing cut off?
3. Compare the critical spots with the original at the same zoom level.
4. If the recipient needs to work with it: search the text, click links and test form fields.
5. For signed documents, note the information further below.

Read the size requirement carefully

A figure in MB can mean 1,000,000 bytes or 1,048,576 bytes per unit. The second reading is correctly called MiB. If the requirement says “at most”, the file may reach the limit exactly. If it says “under” or “smaller than”, it has to stay below. When in doubt, plan with the stricter reading.

So the same file can fit or be rejected depending on the reading. You can convert other values with Convert KB and MB.

Signatures, forms and PDF/A

A scanned signature is an image on the page and is treated like any other image. So check that it stays legible. A digital signature, by contrast, is a cryptographic component of the file. The exported copy is a new file, and this method doesn't guarantee that the signature will still be shown as valid in it. If the recipient expects a digitally signed document, send the unchanged original when in doubt, or clarify the requirement beforehand. There's more on this in the guide PDF with signatures and forms. It's similar with PDF/A: if an organization requires this format, a smaller copy doesn't prove that the requirement is still met.