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I'd be surprised if the built in print to pdf function doesn't already circumvent this.

Google (ab)uses the @media print CSS rule to hide the message content and replace it with "Printing is not allowed by the sender of this message."

However, I've found you can print confidential emails if you comment-out/disable all the @media print rules using Firefox developer tools: https://grokprivacy.org/2018/06/24/archiving-self-destructin...