How to make a PDF smaller on a Mac or an iPhone

Both devices can shrink a PDF, but only one of them has a built-in tool for it, and that tool has a specific weakness: it will not aim at a size. If a portal told you "maximum 2 MB", macOS cannot promise you 2 MB. Here is what each device actually does, and what to do when the built-in route is not enough.

On a Mac: Preview's "Reduce File Size"

Preview has had this for years and it is two clicks away:

1. Open the PDF in Preview.
2. File → Export (not "Save As" — Export is the one with the extra menu).
3. In the Quartz Filter pop-up, choose Reduce File Size.
4. Save under a new name, so you still have the original.

Apple's own documentation is honest about the catch: the exported PDF "may be of lower quality than the original". What it does not say is that you get no control and no target. The filter applies one fixed setting, which produces two familiar disappointments:

• A scanned or photo-heavy PDF comes out much smaller and noticeably softer — often too soft, because the filter downsamples images hard regardless of what they were.
• A text PDF barely moves, or occasionally gets bigger. There were no big images to squeeze, so the filter has nothing to work on.

You can get some control back through ColorSync Utility (Applications → Utilities → Filters tab): duplicate the "Reduce File Size" filter, change its compression quality, and your copy appears in Preview's Quartz Filter menu. It works, but it is trial and error — you still cannot say "2 MB" and be given 2 MB.

On an iPhone: there is no PDF compressor

This is the part worth knowing before you go hunting through Settings. iOS has no built-in "reduce PDF size" function, and the two things people try instead do something else:

• Files → long-press → Compress creates a .zip. That is a different file that the portal will reject, and it usually barely shrinks anything, because the images inside a PDF are already compressed. Zip is for bundling, not for shrinking a PDF.
• The Print → share-sheet trick re-renders the document into a new PDF. Sometimes it is smaller; often it is larger, and it can flatten selectable text into an image, which is a real loss if the file is a CV or a form.

So on a phone the practical route is a browser tool. The 2 MB, 1 MB and 5 MB pages here work in mobile Safari, take the target as the input rather than as a hope, and never upload the file — the work happens inside the browser tab, which matters when the PDF is a bank statement or a passport scan.

Why the same PDF shrinks on one machine and not another

There are only two ways to make a PDF smaller, and they are not interchangeable.

1. A structural re-save. The file is rebuilt: duplicated fonts and images are stored once, orphaned objects are dropped, and the page content is re-packed. Nothing is thrown away, so text stays text and images stay exactly as sharp. On a bloated export from a design tool this alone can cut 30–70%. On a lean file it does almost nothing.

2. Image recompression or rasterising. The pictures are re-encoded at a lower quality, or every page is rendered to an image. This always works — you can hit any target if you push hard enough — and it is lossy. Rasterising also destroys the text layer, which means the PDF is no longer searchable, no longer selectable, and no longer readable by the applicant-tracking systems that parse CVs.

Preview's filter is firmly in category 2. The compressor here tries category 1 first, only moves to category 2 if the target is still out of reach, and tells you which one it used — so you know whether your text survived without having to guess.

Check the text layer before you send it

Open the compressed file and try to select a line of text with the cursor. If you can, the text layer is intact. If the whole page selects as one block, it has been rasterised — fine for a scanned receipt, bad for a CV. This one check catches the most expensive mistake in PDF compression, and it takes two seconds. There is more on this in the CV size post.

When compression is the wrong fix

Sometimes the file is large for a reason that no compressor addresses:

• It has pages you do not need to send. Splitting out the pages that matter is a bigger win than any quality setting, and it is lossless.
• It is a photo of a document taken with a phone. A 12-megapixel photo of an A4 page is a massive image of mostly white paper. Shrink the photo before it becomes a PDF.
• It is scanned at 600 dpi. 300 dpi is enough for anything anyone will read on screen or print at A4.

Why is my PDF so large works through the causes by source — scanner, Illustrator, Canva, Word — if you want to know why yours got big in the first place.

A note on "Reduce File Size" making things worse

If Preview hands you back a file that is bigger than the one you gave it, nothing has gone wrong: the filter re-encoded images that were already efficiently compressed, and re-encoding always costs something. It is a good sign that your PDF was not image-heavy to begin with, and that a structural re-save — or dropping pages — is the route that will actually help.