When cropping helps
Scanned books and slides often come with wide white or grey borders. Cropping them makes the text larger on a phone or e-reader, and pages fit better when you print two to a sheet. It also removes printer marks, headers you do not need or a strip of notes beside a slide.
For scanned books, left and right pages usually have different margins. Crop the odd pages first, download, then open the result and crop the even pages, or use the automatic option, which finds the content on every page separately.
How the crop is saved
The tool sets the visible area of each page (the crop box and media box). Nothing is redrawn, so text stays selectable and sharp at any zoom. The parts outside the frame are hidden, not deleted: they stay in the file and a PDF editor could show them again. To remove content for good, use the Redact PDF tool instead.
Change the page size
Switch to Change page size to turn every page into A4, Letter, Legal, A5, A3 or a size you type in millimeters. Scale to fit enlarges or shrinks the content so it fills the new page without distortion. Keep size centers the content without scaling, which adds white margins or, on a smaller page, hides the edges. Orientation can follow each page or be set to portrait or landscape. Links and comments move with the content.