What does CropBox mean for your PDF?
A page has several boxes: the MediaBox is the sheet, the CropBox is what you see, and print boxes describe trim and bleed. Cropping a PDF usually means shrinking the CropBox.
The content outside is still in the file — hidden, not removed. Anyone can widen the box again and see the margins you cropped away, which matters when the margin held a note, a name or a staple mark.
When the margin content is sensitive, redact it rather than cropping it out of view.
How do you find out what a crop is hiding?
By undoing it, which anyone can do. Cropping narrows the rectangle the reader displays; the content outside is still sitting in the file. Widen the box in any PDF editor, or run the file through a tool that resets it, and the margins come back exactly as they were.
This matters more than it sounds. Margins hold reviewer comments, printed header stamps, names of the person a copy was issued to, and the edge of whatever else was on the scanner bed. A document cropped to hide one of those is still carrying it.
The test is quick: open the file in an editor, look at the page boxes, and see whether the CropBox is smaller than the MediaBox. If it is, something has been hidden rather than removed.
To take margin content out for good, remove it from the page — cropping only changes what is displayed.