I open an old document to edit it. To create space, I have to highlight a few pages of text and move down. Can be a pain if there are many pages.
How can I insert blank space and have the existing move down?

It is not necessary to select and then move downwards. Simply put yourself in the insertion position and click "Enter" a few times to create insertion spaces. With Ctrl+Enter you can close a page and start again with a new one: it can be very comfortable for long documents to obtain a sectioning.
sergio