How the crop works
Enter how much to remove from the top, bottom, left and right, in millimetres. The preview shades the part that will be hidden. Margins are measured from each edge as the page looks on screen, so they behave the same on a page that is stored rotated.
Hidden, not deleted
This sets the page's visible area. Viewers and printers show only the cropped region, but the content outside it is still inside the file and a PDF editor can bring it back. If you need something removed for good, do not rely on a crop.
One setting for every page
The same margins apply to all pages. If your pages are different sizes, check the result: a margin that suits a large page may remove too much from a small one, and the tool will refuse a crop that leaves almost nothing.