What cropping a PDF really does
Cropping a photograph removes what you cut off. Cropping a PDF does something different: it tells readers which part of the page to show, and leaves the rest of the page where it was. That is excellent for margins on a scan and useless for anything you actually want gone, so it is worth being clear about which one you are doing.
Which tool you need
Wide white margins around a scan
Crop PDF. Draw the area to keep on one page and give it to the whole document in one press.
Something on the page that must not reach the reader
Not this. The trimmed material is still inside the file and a reader can restore it. Remove the whole page with Delete PDF pages, or use software built for redaction.
A file that needs to be smaller
Compress PDF. Nothing is thrown away by a crop, so the file stays about the size it was however much you trim.
A page that is on its side
Rotate PDF first. Orientation and shape are different problems, and cropping a page you cannot read the right way up is how the wrong edge gets trimmed.
Pages of different sizes in one document
Crop them together anyway. Applying an area to the whole document keeps the same proportions rather than the same rectangle, so a wider page is trimmed by the same share of its width.
Hidden, not gone
A crop sets the page's visible area. Every reader honors it and the page opens looking the way you left it, but the material outside that area is still in the document and a reader who wants it can bring it back.
That is fine for margins, which nobody wanted anyway. It is the wrong tool for a header with a name in it, a footer with an internal reference, or anything else you are cropping because it should not be seen. For those, the page has to go.
Working on pages that are already turned or trimmed
The preview shows each page the way its own file turns and trims it, and the area you draw is measured against that, so the finished PDF opens at what you saw rather than at something computed from the page's stored shape.
Nothing is committed while you work. Undo and Redo step back through every change, and Remove the trim puts the selected pages back to the area they arrived with, which is the quickest way out of a crop that went wrong on one page.
What to know before you start
- The file does not get meaningfully smaller. Cropping changes what is shown, not what is stored.
- A locked document has to be unlocked first, with its password, before its pages can be trimmed.
- Nothing is uploaded, and your original file is left as it was: the new visible area is written into a copy saved on this device.