Using Grab and scrolling down

I've been trying all day to "print selection" but to no avail. I figured out that the Grab function should be able to help, but when I use Grab "Selection" it will only allow me to select things that are on the screen, and it won't let me scroll down. In other words, the item I'd like to select is larger than one screenshot, and so when I use Grab and select the portion of the screen I'd like and then move the cursor to the bottom, rather than scrolling down, the screen does nothing. I am only able to select half of what I'd like! Would appreciate any help. Thanks very much!