Selecting half a document
If the text is at the second half of the document, go to the beginning of the text You want to delete and press ⌘ CMD+⇧ SHIFT+↓ DOWN ARROW and then delete it.
If it's in the first half, go to the beginning of the text You do not want to delete and press ⌘ CMD+⇧ SHIFT+↑ UP ARROW and delete it.