Preview can do it: File, then Export, then Quartz Filter, then Reduce File Size. Apple's own note on that page says the result may be lower quality than the original and that how much smaller it gets varies.
What it will not do is tell you how small the file will be, or let you aim at a number. For a document that has to fit under a limit, that is the whole problem.
Most people reach for this because something has to be emailed and a portal has rejected it. Preview is already installed, which makes it the obvious first try, and it is worth knowing exactly what it does before you send the result to anybody.
The built-in route
Open the PDF in Preview
Double-clicking a PDF on a Mac opens it in Preview unless you have changed the default.
File, then Export
Not Save As and not Export as PDF. Export is the one with the filter menu in it.
Quartz Filter, then Reduce File Size
The pop-up menu is near the bottom of the sheet and says None until you change it.
Save, under a new name
Preview will offer to replace the original. Give it a different name instead, so you still have the file you started with.
That is the whole of it, and Apple's own page for the feature is a short read. The line worth noticing is theirs, not ours: the PDF may be of lower quality than the original, and the resulting size and quality vary with the options you pick.
Why it sometimes makes the file larger
A Quartz filter is a ColorSync recipe. Reduce File Size applies the same recipe to every document it is given: downsample the images, re-encode them, write the file out again. It does not look at what is in the document first.
So the rule of thumb is simple. A PDF made of scans or photographs will shrink, often dramatically. A PDF made of text, tables and line drawings has almost no image data in it, and the filter has nothing to work with.
The ColorSync trick, and why to skip it
Because Quartz filters are just files, people have been writing their own since the days of Tiger. ColorSync Utility, which lives in the Utilities folder, will let you duplicate the Reduce File Size filter and change the resolution and JPEG quality it uses, and the new filter shows up in Preview's menu.
What else a Mac already has
Preview's Reduce File Size, for image-heavy documents.
Print, then Save as PDF, which can drop some structure and does not reliably shrink anything.
A custom ColorSync filter, if you are willing to tune numbers.
Automator, if you want the same filter applied to a folder at a time.
Aim at a size. Nothing built in lets you name a target.
Tell you what it will cost before you do it.
Leave the text alone selectively; the filter treats the document as one thing.
Help at all with a document that has no images in it.
Automator deserves a word, because it is the underused one. A Quartz Filter action in a Folder Action will apply the same filter to anything dropped into a folder, which is genuinely useful if you compress the same kind of document every week and have already found a filter that suits it.
Checking what you actually got
A compressed PDF is a new document, and it is worth thirty seconds of looking before it goes anywhere. Three things catch most problems.
Open it, and count the pages
A filter should never change the page count. If it has, something has gone wrong further back and the file was probably already damaged.
Zoom a photograph to 100 per cent
This is where downsampling shows. At the size Preview displays a page by default, an image at a quarter of its resolution looks fine; at full size it does not.
Check the text is still text
Select a paragraph. If it selects word by word, the text survived. If nothing selects, the document was rasterised somewhere in the chain and it is no longer searchable.
Look at the size before you send it
The whole point was a number. Get Info in the Finder, or the size column in list view.
That third one matters more than people expect. A PDF whose text has become a picture is larger, unsearchable, useless to a screen reader, and impossible to copy from. Preview's filter does not do that, but a few of the routes people try when the filter disappoints them will.
When you need to hit a number
This is where the built-in route runs out, and it is the common case: a form says nothing over 2 MB, or an email bounces at 10. What you want then is not a recipe but a search, trying a quality setting, measuring the result, and adjusting until the file lands under the limit.
It is also worth knowing which direction to push. Most of the size in a typical document is images, and most of the rest is embedded fonts. Text and vector drawings cost almost nothing. A twenty page report that is 12 MB is 12 MB of photographs, and the only question is how much resolution they can lose before it shows.
That is what the compressor here does, and because it runs in the browser the document stays on the Mac. It shows what each page costs before you commit, and there are pages for the common targets if you want to start from one: 1 MB, 2 MB, under 10 MB.
Compress a PDF to a size
Runs in Safari or Chrome on the Mac. The file never leaves the machine.
Common questions
The built-in route, its limits, and the times it makes things worse.
01Where is Reduce File Size in Preview?
File, then Export, and then the Quartz Filter pop-up menu near the bottom of the sheet. It says None until you change it. It is not under Save As, and it is not in the Export as PDF item, which is the one people usually find first.
02Why did my PDF get bigger after compressing it?
Because the filter applies the same treatment to every document. If yours was mostly text with a small logo, there was very little image data to downsample, and re-encoding what was there and writing the file out again cost more than it saved. Files made of scans and photographs are the ones that shrink.
03Can I choose how much it compresses?
Not in Preview. You can duplicate the filter in ColorSync Utility and change the resolution and JPEG quality it uses, and the copy appears in Preview's menu. It is still a fixed recipe rather than a target.
04Does compressing in Preview lose quality?
Yes, and Apple says so on the page for the feature. The images are downsampled and re-encoded. How visible that is depends on what the document is and how it will be used, which is why exporting under a new name matters.
05Is there a way to compress several PDFs at once?
Automator. Build a Folder Action with an Apply Quartz Filter to PDF Documents step, point it at a folder, and anything dropped in gets the same treatment. Test it on copies first, because a Folder Action modifies files in place.
06Do I need to install anything?
No. Preview and ColorSync Utility are already on the Mac, and a browser tool needs nothing installed either. Compressing a PDF is not a reason to add software to a machine.