There are various reasons why you might want to compare two documents and Word provides the option to open two files side by side for this very purpose. However, if you're using a single monitor in portrait mode, side-by-side document comparison is less useful than having one document displayed above the other.
Fortunately, the option to arrange two documents side-by-side is available. In Word 2003 or older, open the two documents you want to compare, then click on the 'Window' menu and select 'Compare Side by Side'. Now click the 'Window' menu again and select the 'Arrange All' option to orient them like that.
Even better, if you want to scroll through both documents at the same time, you can. In Word 2007 and 2010, open the documents that you want to compare and move to the View tab of the ribbon. Click the 'View Side by Side' button, and then click 'Arrange All'. Press 'Synchronous Scrolling' and you can scroll simultaneously through both documents