This is how krop looks after opening a PDF file and selecting a region. In fact, the region you see here was automatically selected by pressing
Using the advanced options ...
... allows us to split pages into subpages to fit the limited screensize of, say, a Nook eReader with a screen size of 730x600 (when held upside down). Note how the selection indicates that it will be split into two pages. Thus, the resulting cropped PDF file will have 38 instead of 19 pages, with each fitting the screen of the eReader perfectly; no zooming or scrolling required.
You can create any number of selections per page, each of which will be a separate page in the cropped PDF. In the example below, two selections were made and each is (automatically) subdivided because of the option to fit the screen of an eReader with a 4x3 aspect ratio. The cropped PDF will therefore have 32 instead of 8 pages.