Take a look at this old question: "how to put permission to only paid users can print something in my drupal site".
It is from April last year, and has an accepted answer with four upvotes. It was closed about ten months after it was was asked (FTR: I voted to leave open, and now it is open again).
I am very much in favour of closing bad, old questions that does not have an accepted answer. That pesky community user keeps re-inserting such questions on the front page, and if it is a bad question - that is just noise.
But if the question already has an accepted answer, and it wasn't closed when it was fresh - it could have not been that bad. After all, somebody were able to answer it and the answer solved the problem for the OP.
And maybe a new module, or something else, comes along that may give grounds for an even better answer in the future? In this case, the Adobe's PDF format happens has a built-in DRM-feature that is easy to integrate with Drupal that isn't mentioned in the answer given. Adobe's built-in DRM will do exactly what the OP want. If the question is closed, that answer can't be given.
I suggest we leave these alone, in particular if they're very old. To nominate those for closure just makes the close queue longer and prevents updated answers from being posted.