Trim the margins, cut away a scanner's black border, or bring the content closer to the edge — on every page or only on the ones you choose.
Cropping changes the visible area of a page. It is ideal for scans that came out with a wide grey border, slides with too much white space, or a document whose content is squeezed into one corner by a bad scan preset. Cropping does not delete anything from inside the visible area, and it does not resize images.
Text or images outside the crop box may still be present in the file and extractable by someone who edits the crop back to its original size. If the goal is to remove information rather than to tidy up appearance, use Redact, which strips the content itself.
A crop that reads well on page 1 can cut text on page 2 if the margins differ — which they do in any document assembled from several sources. Crop one page, check, then apply to the rest.
Not much: the hidden content is usually still there. Use Compress for size.
Run the tool once per group of pages, producing one file per crop.
Usually yes, by changing the crop box back. Do not rely on cropping to hide data.