Crop PDF

What cropping a PDF really does

Every PDF page carries several boxes. The MediaBox is the full sheet the page was made on. The CropBox is the part a viewer shows and a printer prints. Two more, the TrimBox and BleedBox, matter to commercial printers: the finished size after trimming, and the extra area that runs off the edge.

Cropping sets the CropBox. The content outside it is not deleted — it is simply no longer shown, and any PDF editor can reset the crop and bring it back. You can see this for yourself: crop a page here, then use Restore the full page on the cropped file, and everything returns.

That makes cropping the right tool for presentation and the wrong one for privacy. To get rid of an account number in the margin or a name in a header, use Redact PDF, which rebuilds the page so the characters no longer exist.

Common reasons to crop

Reading on a phone, tablet or e-reader. Academic papers and books are often set with wide margins for printing; trimming them makes the text noticeably larger on a small screen, because the viewer scales the cropped page to fill the display.

Cleaning up scans. Flatbed scans pick up a dark border, a ruler or part of the scanner lid; phone scans pick up the table. Crop them away and the document looks deliberate.

Cutting out one thing. A chart, a diagram or a signature block can be isolated on its own page — crop to it, then extract the page if you need it as a separate file.

Removing printer's marks. Print-ready PDFs often include crop marks and colour bars outside the trim area, which look odd on screen.

Precise margins, and the auto-detect button

The drag box is quick; the margin fields are exact. Type top, bottom, left and right in millimetres, inches or points — a point is 1/72 of an inch, so 1 inch is 72 points and 10 millimetres is about 28.3 points. The two always agree: drag and the numbers change, type and the box moves.

Auto-detect margins renders the page at a low resolution, finds the colour of the background from the corners — white for most documents, a grey or cream tint for many scans — and works out the smallest box that contains everything else, ignoring isolated specks of dust. The padding you set is kept around the content so the crop does not shave the edge of a letter.

Margins are measured from the edges of the page as you see it. On a page that is stored sideways and rotated for display, “top” is still the top of the screen, which is what you meant.

One page, many pages, and pages that differ

The same margins can be applied to the current page, all pages, or a range such as 2-10. Margins are distances from the edges, so they mean the same thing on pages of different sizes.

When the content sits in different places on different pages — slides of varying layouts, a scanned book where the text drifts — tick Detect the margins of every page separately. Each page is then analysed and cropped to its own content.

Some printing and layout programs ignore the CropBox and use the full MediaBox. If a cropped file comes out uncropped in one of them, tick Also shrink the page itself: the MediaBox is set to the crop as well, and the page's other boxes are kept inside it. The content outside is still in the file, but every program now treats the cropped size as the page size.

Cropping is not resizing

Cropping removes the edges of a page; it does not scale what is left. A page cropped from A4 to a 150 × 200 mm box is a 150 × 200 mm page with the content at its original size, which is what you want for reading on a screen or cutting out a figure.

If you need the result to be an exact size — a page trimmed to A5, say, which is 148 × 210 mm — type margins that leave that size. The line under the page shows the size the crop will produce, in the units you chose, as you type. To fit a page onto a different sheet of paper instead, with the content shrunk to match, use your printer's fit-to-page setting or Multiple Pages Per Sheet; cropping is the wrong tool for that job.