Those who down-vote: could you please explain why you down-vote? What rule am I against so that you down-vote my question? Rather: give a reasonable answer...

Yes, I went up one level to meta.stackexchange.com. It is actually weird that this has been proposed and got highly upvoted and received positive comments, but never got implemented...

Well, I am not talking about copyrighted content. E.g. someone in his answer posted this link recently. This is a great PDF and there is valuable information there that should be made available permanently. Sometimes we might need a permission of the owner, but many of those PDFs are free for all use if they are kept intact.