Got this information from our ad ops.
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Google is experiencing a large scale glitch in ad creative rendering impacting Google DV360 and Adsense. This has been happening since Tuesday September 22 early morning. The issue manifests as a browser dialog box to confirm the download of a file. The file is -not- malicious, this does -not- contain malware, just ad serving code.
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https://news.thewindowsclub.com/firefox-chrome-edge-safari-forced-to-download-mysterious-f-text-file-103254/
These f.txt is not malware according to this recent report/article that came out today. It is a glitch with Google ads that is being investigated.