Crop a PDF

What cropping a PDF actually does

It changes the page boundary, not the content. Every viewer, printer and phone then shows and prints only what is inside the new boundary — which is what you want, and it is worth knowing that the material outside it is still in the file rather than deleted.

That distinction matters in one specific case: cropping is not redaction. If the margins contain something confidential — a header, a watermark, a name — moving the page edge hides it from view while leaving it recoverable by anyone who changes the boundary back. For that job use redact PDF, which removes the content itself.

Where the wasted space comes from

Two common sources. A scanner set to A4 photographing a smaller book page, which leaves a broad white surround on every page. And an academic paper or ebook typeset with generous print margins that are simply wasted on a screen.

Trimming both cases makes a real difference on a phone or an e-reader, because the reader scales the page to the screen width — so a page that is 30% margin renders the text 30% smaller than it needs to be. Cropping the margins is the single most effective way to make a PDF readable on a small screen.

Measure once, in points

The four values are in PostScript points, where 72 points is one inch and an A4 page is 595 by 842. So 36 points is half an inch, and 72 is a full inch.

The way to get this right first time is to open the PDF at 100% and read off how much white you want gone. Guessing tends to go one of two ways: too timid to be worth doing, or far enough to clip descenders and page numbers, which you only notice pages later.

Crop some pages, not all

A page range is worth using because documents are rarely uniform. A scanned book alternates left and right pages with the wide margin on opposite sides, so the same crop applied to all of them takes the gutter off one side and the text off the other. Do the odd pages, then the even pages, with the left and right values swapped.

A cover or a full-page figure usually wants leaving alone entirely.

Undoing it

Because the content is still there, cropping really is reversible — a genuine advantage over cropping an image, where the pixels are gone. Tick put the page boundary back and the page returns to the full sheet: measured, a page cropped to 40 points and then reset came back to its original 595 by 842.

One thing worth knowing, because it is the opposite of what most people assume: zeros on their own do not undo a crop. The trim is measured from the boundary the page currently displays at, so on an already-cropped page zeros mean “change nothing”. That is also what makes cropping twice behave sensibly — 40 points and then 10 more gives you 50 — rather than the second crop being measured from an invisible larger sheet underneath.

It is also the reason for the redaction warning above: reversible cuts both ways.

If the crop lands in the wrong place

Some PDFs already carry a crop box smaller than their media box, most often after being processed by other software. The trim here is measured from the boundary the page is actually displayed at, which is what makes the result match what you see on screen rather than an invisible larger page underneath it.

That distinction is not theoretical. Measured on a page whose crop box was already inset by 50 points, a 20-point trim taken from the media box edge landed at 20 instead of 70 — removing less than a third of what was asked for, and looking exactly like the tool had ignored you.