Thank you for these suggestions. LoyceV's PM Publisher is good but as I understood, you have to use it the moment you send a PM, it's not usable if you already sent a PM to someone else. I also don't want my PM to become published and readable by others. I simply want to have a proof that I received a certain PM from one user. I can prove that by logging in to my Bitcointalk account but the problem is, what if Bitcointalk goes down? How can I prove my PM? It's a little bit complicated and I can't talk in deep detail.
Btw can you explain to me how Ninja's Bitcointalk PMs work?
There is an easy and simple way to save messages from your web browser.
First select messages you want to save, than go to your browser menu and select Print, and Save it in PDF format.
This should work both in Chrome, Firefox and in all other forked browsers.
You can fit more pages per sheet, remove headers, and remove headers.
That's not the best proof because that part can easily be modified. If you archive a forum post via web.archive.org, that's great proof that a certain post was really posted on this forum because no one can modify what web.archive.org archived. I'm looking for a similar solution but for PM.