Trim the margins of a PDF
Cropping is most useful on documents that were produced for paper and are now being read on a screen. A scanned book page surrounded by two inches of white space wastes most of a tablet display; trimming it makes the text usably large without zooming and panning on every page.
It is also the fix for a scan that picked up the edge of the platen, or a slide deck exported with printer margins it does not need.
What cropping actually does — and does not do
Every PDF page has two rectangles that matter here. The media box is the physical sheet. The crop box is the region a reader should display. Cropping sets the crop box; it does not touch the media box and it does not remove anything.
That has one good consequence and one dangerous one. The good one: the operation is completely lossless and completely reversible — no pixels are resampled, no text is re-flowed, and resetting the box brings the full page back. The dangerous one: content you crop away is still in the file, and anyone can recover it.
If your reason for cropping is that something in the margin should not be seen, this tool cannot help you, and neither can any crop that only moves the visible box. Removing the whole page with Remove PDF Pages genuinely deletes it; cropping here never does.
Choosing margins
Margins are given as a percentage of each dimension, so the same setting behaves sensibly across a document that mixes A4 and letter, or portrait and landscape.
Five percent is a light trim that removes obvious white space. Ten to fifteen percent is typical for a scanned book. Beyond about twenty-five percent you are usually cutting into content, which is why the preview shows the proportion of the page that survives.
Related tools
To turn pages rather than trim them, use Rotate PDF. To make the file genuinely smaller, use Compress PDF. To delete whole pages, use Remove PDF Pages.