Uncrop

You can crop any part of a pdf. When you crop it, Preview actually keeps track of the part of the document you deleted and hangs on to it in case you want to uncrop the file, even after you've saved it.

Source: wiktionary

In other words, if you rotate a photo, crop it, and then change its contrast, you have to use the Undo command three times—first to undo the contrast change, then to uncrop, and finally to unrotate.

Source: wiktionary