How to crop a PDF, and what cropping does not remove

Every page has two sizes

A PDF page records its dimensions in a MediaBox: the physical sheet, the thing a printer would put paper to. It can also record a CropBox, a smaller rectangle inside it, meaning show this part. When the two differ, a reader draws the CropBox and you see a smaller page.

Nothing is thrown away to make that happen. The text, the images and the vector art outside the crop are all still objects in the file, in their original positions. They are outside the window, so they are not painted.

This is why cropping is instant even on a two-hundred-page document, and why it never degrades quality: no pixel is resampled and no character is re-encoded. It is also why the three things people expect to follow from cropping — privacy, a smaller file, a different paper size — do not follow from it at all. Each of those has its own section below.

What cropping is genuinely good for

Four jobs, all of which are about presentation rather than content:

• Scan edges. A page photographed or scanned at a slight angle arrives with a grey or black border and part of the facing page. Cropping removes it from view, and the document stops looking like a scan.
• Oversized margins. Academic PDFs and documents built for A3 waste a great deal of screen on a phone. Tightening the crop makes the text larger without changing a font.
• Slides. Decks exported to PDF often carry a border, a footer bar or a presenter frame that is pure noise in a document.
• Trimming to one region. A single chart or table on a busy page, cropped so it can be dropped into something else.

Crop PDF applies one rectangle across the pages you choose. Where pages differ — a landscape chart in the middle of a portrait report — crop those separately, because one rectangle that suits both usually suits neither.

Cropping is not redaction, and the difference matters

This is the one thing worth taking away from this page. Cropped-out content is still in the document. It is not visible, and it is trivially recoverable.

Three ordinary things bring it back:

• Selecting and copying. Many readers copy text from the full page, not the visible window. Select all, paste into a text editor, and the cropped text appears.
• Removing the crop. Setting the CropBox back to the MediaBox is a one-step operation in any PDF editor. Nothing is being broken into; the file is simply being asked to show what it holds.
• Extracting the text layer. Anything that reads the page's content stream, including PDF to text, sees every character on the page regardless of where the window sits.

If the point is that somebody must not be able to read something, cropping is the wrong tool and so is drawing a black rectangle over it. Use Redact PDF, which removes the content rather than covering it, and read how to redact a PDF permanently first — the failure mode there has ended careers, and it always looks fine on screen.

The same caution applies to the parts of a file nobody looks at. Cropping does not touch document metadata, so the author name and the original software survive it unchanged.

Why the file does not get smaller

A cropped page looks like a third of the page it was, so the file ought to be a third of the size. It is almost exactly the size it was.

Everything outside the crop is still stored. A full-page scan is one large image object, and cropping the page does not cut the image up — it draws the same image and shows part of it. The bytes are untouched.

If size is the actual goal, crop for appearance and then deal with size separately: Compress PDF works on how the images are stored, which is where the weight is. Why some PDFs cannot be compressed explains which files have room to give and which do not.

One exception worth knowing: some tools offer to flatten or re-render cropped pages into new images. That does discard what is outside the crop, and it also discards the text layer — the page becomes a picture, and from then on nothing can search it, copy from it or convert it. That is a real trade, and it should be a deliberate one rather than a side effect.

Cropping, paper and printing

A cropped page is not a resized page. The sheet is still whatever it was; only the window moved. Send a cropped A4 document to a printer and most drivers will scale the visible area onto A4, leaving wider margins than you expected, or print it at its original size with the crop ignored entirely. Which of the two depends on the driver, which is a bad thing for an outcome to depend on.

If the document needs to be a particular paper size, set the size rather than the window: Resize PDF changes the page itself. PDF page size and orientation covers how A4 and Letter differ and why a page can look right on screen and print wrong, and getting a PDF ready to print covers what to check before a file goes to a commercial printer.

A short rule that holds in practice: crop for reading, resize for printing, redact for secrets.