DRM to prevent opening or reading a file has absolutely nothing to do with this issue.
You'd have to encrypt the file to prevent unauthorized people from reading the contents.
That's the only way to enforce DRM.
This issue is about copying only. And preventing copying is, at least for text, simply impossible. If somebody wants to copy the text, the person can always re-type it. Even for diagrams, I can just take a screenshot.
All this flag does is making things more inconvenient.