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Cropping a PDF sets its crop box, which tells a reader which part of each sheet to display. It does not delete anything: the content outside the box is still in the file, and widening the box again brings it back. That is worth knowing before you crop a signature or an account number off something and send it on.

The usual reasons are a scanner that left 15 mm of grey around every page, a document with margins so wide it wastes half a tablet screen, a page that has to fit a slide or an e-reader, or receipts photographed with a desk in the frame.

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1. Drop the PDF onto this page and wait for the first page to render.
2. Choose the mode: drag the rectangle yourself, let the tool find the content, or crop to a fixed shape.
3. For a manual crop, drag the handles, or type what to take off each edge in millimetres, points or inches.
4. For an automatic crop, set the breathing room to leave around the content - six points is a sensible start.
5. Decide whether to crop every page or only a range, then press Crop.
6. Check a page in the downloaded file before you discard the original.

The automatic mode is why most people come here. Each page is rendered small - 400 pixels on its long edge, which is ample, since ink boundaries do not move when you look at them from further away - and every pixel more than about 9 percent away from white counts as content. The bounding box of that content, plus your margin, becomes the crop box. Rendering 300 pages at full resolution to find their margins would cost more than the crop.

Automatic cropping measures each page separately, so pages end up different sizes when their content does. That suits reading on a screen and is awkward for printing, where one tray wants one size; crop by hand or by shape if every page must match. A page with no ink has no box to compute, so it keeps its full size rather than being cropped to nothing, and a document where every page is blank is refused rather than destroyed.

Crop to a shape takes the largest rectangle of the proportions you pick that fits inside the page, centred. A4 is the 1:1.414 shape of the paper itself, so it trims a letter-sized page to those proportions; 16:9 on a portrait page removes a great deal of height, which is the point when the result is going into a slide. Nothing is scaled - only the visible window changes.

Also shrink the page size reduces the media box along with the crop box, so readers and printer drivers that ignore the crop box obey too. It is harder to undo, and it is still not deletion: the content outside is exactly where it was.

Everything happens in your browser: PDF.js renders the pages to measure them and pdf-lib writes the boxes. The pages are never re-encoded, so a scan keeps its original image data at full resolution.