One can also permanently archive the offer, if it has a unique URL, with the following website:
http://archive.today
While previously based in Israel and Vietnam, for me it currently (perhaps based on my geographic location) resolves to:
https://archive.ph/
The resulting evidence is independent and contemporaneously verified, unlike screenshots and PDFs that could theoretically be manipulated.
The archive links are also easier to share with frequent flyer / guest programs, banks, regulatory authorities, and fellow FlyerTalkers -- by posting or private message (PM).
We would all do well to make use of this free resource.