Crop a PDF

Cropping trims the margins off every page (the scanner's black border, the slide deck's dead space, the whitespace around a receipt), so the part you want fills the page. Set the crop whichever way suits the document: type how much to shave off the top, right, bottom, and left, in millimetres, inches, points, or a percentage of the page; drag a box over the preview instead; or let Auto-trim measure each page and take the whitespace around whatever is printed on it. How much of the document one setting covers depends on which way you picked: a margin trim is a single figure per edge applied to every page, Draw a box either repeats one rectangle across the document or takes a separate box drawn on each page, and Auto-trim measures every page on its own, so a page ringed by wide whitespace loses more than a page that is nearly full. A page range narrows which pages are cropped at all, and the preview updates as you go, so you can dial it in before you download.

It helps to know what a crop is inside a PDF. The format's own crop is a viewing instruction: a /CropBox tells the reader which window of the page to show, and everything outside that window is still in the file, one setting away from coming back. Clipping the page behind a smaller frame comes to the same thing: the trimmed part is hidden, not gone. That is exactly what you want when you are tidying margins, and exactly what you do not want if you cropped something off because you did not want it seen.

So the default here is to actually delete it. Pick "Delete the cropped-out area" and each cropped page is re-rendered and rebuilt from the kept region only: the trimmed pixels are never drawn, never encoded, and are not in the file you download. Before the file reaches you it is checked: every page that was cropped has to come back with no extractable text left on it, or the crop fails and you get nothing rather than something that merely looks trimmed. The trade is that those pages become images, so they lose selectable text and links and the file gets bigger.

The other two settings are cosmetic by design. "Keep text selectable" gives you vector output with working text and links. "Keep links and form values" narrows the visible page instead: links keep working and filled-in values stay visible, though the fields themselves generally stop being editable, since rebuilding the file drops its form dictionary. In both, the trimmed area is hidden rather than gone, and the tool says so where you choose. All three crop exactly the same rectangle; they differ only in what survives. Everything runs in your browser, and the original on your disk is never touched.