Trim the pages of a PDF down to a selected area. Drag out the region you want to keep and everything outside it is cut away. Typical uses are removing the heavy margins on a scanned book so the text fills more of the screen on a tablet, or cutting off the black edge a scanner leaves when the original was smaller than the platen.
Cropping sets the visible page box, and content outside it is no longer displayed or printed. If the material outside the crop is sensitive, do not rely on this - use Redact, which removes the underlying content.
The selected area is applied across the document, which works well for a scan where every page sits in the same place. A document with alternating left and right margins is better cropped in two passes over the odd and even pages.
The selection has gone outside the page rectangle, which can happen after zooming. Redraw the area so it sits entirely within the page.
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