How to compress a PDF on Mac without uploading it anywhere

Almost every “compress PDF” result is a website. You pick a file, it uploads, a progress bar runs, and you download something smaller. It works — and it also means a contract, an invoice or a medical scan just sat on a server you know nothing about, under a retention policy you did not read.

If the document is boring, that is a fine trade. If it is not, here are three ways to shrink a PDF on a Mac where the file never leaves the machine.

1. Preview’s built-in filter — free, instant, and often too aggressive

macOS ships with a compression filter. It is hidden in an odd place:

1. Open the PDF in Preview.
2. File → Export… — not Save As, and not Export as PDF. Only plain Export shows the filter menu.
3. In Quartz Filter, choose Reduce File Size.
4. Save under a new name.

That filter really does live on your Mac — you can see it:

ls /System/Library/Filters/ | grep Reduce
# Reduce File Size.qfilter

The catch is that it has no settings. Apple’s filter downsamples every image to a fixed resolution and re-encodes it at a fixed JPEG quality, whatever the document is. On a text-heavy report you may see 80% off with no visible difference. On a portfolio, a scanned plan or anything with photographs, the result is frequently unusable — soft type, blocky gradients, and diagrams that have quietly lost their fine lines.

There is also no way to tell in advance which of those two you are about to get. You find out by looking.

2. Build your own Quartz filter — free, fiddly, permanent

Because the filter is just a file, you can make a gentler one. ColorSync Utility (in /Applications/Utilities/) has a Filters tab where you can duplicate Reduce File Size and change the image sampling resolution and compression quality by hand. Save it, and it appears in Preview’s Export menu alongside Apple’s.

This is the best free answer, and it is genuinely useful if you compress the same kind of document over and over. Two honest caveats: the interface is a relic that has barely changed in fifteen years, and you are still exporting one file at a time.

3. Batch compression with a quality slider

The real problem with both routes above is not quality. It is that a Mac compresses one PDF per trip through a dialog, and the month-end folder has forty of them.

ByEase File Tools exists for that shape of work. Drop a folder in, set the quality once, and every file is processed in place — with a slider so you decide the trade-off instead of accepting Apple’s fixed guess. Nothing is uploaded: there is no server component in the app at all, which is a stronger guarantee than a privacy policy, because there is nowhere for the files to go even in principle.

It also runs from the Finder context menu, so the whole operation is: select the files, right-click, Open in ByEase.

Which one to use

• One document, mostly text — Preview’s filter. It is free and already on your Mac.
• The same document type every week — build a Quartz filter once in ColorSync Utility and reuse it.
• A folder at a time, or anything with images you care about — use a tool with a quality control and batch input.

A note on what compression actually does

Nearly all PDF size lives in the images. Compression re-encodes those images at a lower resolution or a lower JPEG quality; it cannot make text smaller in any meaningful way. Two practical consequences:

• A PDF that is genuinely all text and already small will barely shrink. If a 40 KB document is not compressing, nothing is broken.
• Compression is lossy and one-way. Always keep the original — every route above should write to a new file, never overwrite in place.

If the goal is only to slip under an email attachment limit, splitting the document is often the better move than crushing its images. That is a different job, and a different guide.