A watermark is a faded background image that displays behind the text in a document. You can use them to indicate a document’s state (confidential, draft, etc.), add a subtle company logo, or even for a bit of artistic flair. Here’s how to add and manipulate watermarks to your Word document.
How to Insert a Built-In Watermark
With your document open, switch over to the “Design” tab.
In the Page Background group on that tab, click the “Watermark” button.
On the drop-down menu, click any of the built-in watermarks to insert it into your document.
Word places the watermark behind the text.
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Source: How-To Geek