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On the Design tab, select Watermark.
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Choose the watermark you want.
Tip: If the watermark appears only on the first page, or is missing on the first page, try going to a page where the watermark is missing, and repeat these steps.
Make a watermark from a picture or custom text
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On the Design tab, select Watermark.
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Select Custom Watermark, and then select Picture or Text.
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Add the picture or text you want.
Source: Microsoft