What the screenshot you provided shows is an internal label used by Microsoft to describe how Word is handling the document behind the scenes. Under normal circumstances, users never see this text at all.
The fact that it appears as a white strip at the very bottom of the page confirms this is a display/rendering glitch, not something typed into the footer, header, or body of the document. It is also not malware, not a hack, and not related to your files being uploaded anywhere. “Server Only” refers to how Word’s engine is designed, not to your computer sending documents to a server.
This kind of issue typically appears after a recent Word or Microsoft 365 update, or when Word’s graphics rendering briefly misbehes. That also explains why you only see it in Word and not in Excel or PowerPoint — they use different rendering systems. Microsoft has had similar bugs in the past where internal debug text briefly leaked into the document view and was later corrected in an update.
Importantly, this text will not print, and in most cases other people will not see it if you send them the document. It’s tied to how Word is drawing the page on your screen, not to the document itself.
In practical terms, this usually resolves itself by restarting Word, restarting the computer, or after the next Office update.
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hth
Marcin