everyday mac tools

Crop a PDF on a Mac, and why the cropped part is still there

· 6 min read

To crop a PDF on a Mac, open it in Preview, drag a rectangle over the part you want to keep, and choose Tools, Crop (Command-K). To crop every page the same way, select all the page thumbnails in the sidebar first. What Preview does not do is delete anything: the area outside your rectangle is hidden, not removed, and it stays in the file for anyone with the right software to bring back.

For trimming scanner margins or white space, that does not matter. For cutting off someone’s address, an account number or the other half of a page, it matters a great deal. Both cases are below.

Crop one page in Preview

  1. Open the PDF in Preview. If the original is important, choose File, Duplicate first and work on the copy.
  2. Choose Tools, Rectangular Selection. It is usually selected already.
  3. Drag a rectangle around the part of the page you want to keep. Drag the handles on its edges to adjust it.
  4. Choose Tools, Crop, or press Command-K.
  5. Preview may warn you that cropping does not delete the content outside the selection. Read it, because it is the most important sentence in this whole process, and click OK.

The page now shows only the area inside your rectangle. Command-Z undoes it while the document is open.

Crop every page the same way

Scanned documents often need the same margins trimmed from every page.

  1. Choose View, Thumbnails to show the sidebar.
  2. Click one thumbnail, then press Command-A to select them all. To crop only some pages, Command-click the ones you want.
  3. Drag your rectangle on the page in the main view.
  4. Choose Tools, Crop. Preview applies it to every selected page.

Scroll through afterwards. If the pages of a scan were fed in slightly crooked, one rectangle that suits page one can clip text on page twelve. Crop those pages separately, or leave a more generous margin on all of them.

Why the cropped part is still in the file

A PDF page has more than one idea of its size. There is the full page as it was created, and there is a crop box, which tells viewers which part of that page to display. When Preview crops a PDF, it changes the crop box. The text, drawings and images outside it are left exactly where they were.

A few consequences follow:

  • The file does not get smaller. Cropping a scanned page leaves the whole scanned image inside; the viewer just stops showing some of it. If you are cropping to save space, it will not.
  • Other software can undo it. Many PDF editors can reset the crop box and show the full page again, and it takes seconds.
  • Text outside the box can often still be extracted. Copying and text-extraction tools generally read the page’s content, not just the visible window onto it. A name you cropped off the bottom of a letter may turn up when someone selects all and pastes.

So cropping is a layout tool, not a privacy tool. It is the same mistake as drawing a black box over text, just harder to spot. Why a black box on a PDF does not redact anything explains the general problem.

When the cut-off part must not be shared

If anything outside your crop is private, remove it before you crop.

Redact it first in Preview. Tools, Redact, then drag across the text or the area you want gone, and save. Preview removes the text and image content under each mark. Then crop, so the page looks the way you want. Redacting a PDF in Preview covers exactly what Preview removes and where it stops short, including metadata and the hidden text layer in a scanned PDF.

Or turn the visible area into a picture. For one page, you can crop, then choose File, Export, pick PNG or JPEG, and save. The exported image contains only the pixels you could see. Open it in Preview and export it as a PDF again if you need a PDF. The result has no selectable text, which is the point, and also the cost: searching and copying stop working unless you add a text layer back later.

Check the result, not the process. Open the final file, choose Edit, Select All and Copy, and paste into a TextEdit document. Nothing from the cropped or redacted area should appear. How to check whether a PDF was really redacted lists more checks, including searching the file for the exact words you removed.

For documents where this is a regular job, Basalt is a Mac PDF app built around redaction that destroys content instead of covering it. Text under a mark is removed glyph by glyph and images under a mark are destroyed and re-encoded; metadata, attachments, hidden layers and stale thumbnails are stripped. Before any file is written, a verifier re-opens the export with an independent parser and confirms the strings are absent and the pixels changed; if a check fails, no file is written. Its Inspect tool shows what a PDF still hides, and its page tools handle reordering, rotating and deleting pages. Every tool writes a new file and leaves the original alone. It is $29 for a lifetime license on up to three Macs, free for 24 hours with every tool.

Cropping for layout: the useful cases

None of this makes cropping bad. It is the right tool when the cut part is simply unwanted, not secret:

  • Trimming wide scanner borders so a page fills the screen on a tablet.
  • Cutting a slide deck exported with large margins down to the slide.
  • Tidying a receipt scanned on a full letter-size page before you file it.

If you later regret a crop, choose File, Revert To and pick an earlier version while you are still on the Mac where you edited it. macOS keeps versions of documents you change in Preview, which is useful here, and a reminder that the uncropped version is on your Mac too.

Questions

Does cropping a PDF in Preview reduce the file size? No. The content outside the crop stays in the file, so the size barely changes. To shrink a PDF, use Preview’s File, Export with the Reduce File Size filter, or a dedicated compression tool.

Can someone see the part of a PDF I cropped out? Often, yes. Many PDF editors can reset the crop, and text-extraction tools may still pick up words from the hidden area. Redact anything private before cropping.

How do I crop all pages of a PDF at once on a Mac? In Preview, show thumbnails, select all of them with Command-A, draw your selection on the page and choose Tools, Crop. Every selected page gets the same crop.

Can I undo a crop after saving? In Preview, File, Revert To usually has an earlier version. Once the file has been sent, the recipient’s copy still holds the full page anyway, so there is less to undo than it seems.