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I think most of the security bits require 5 or later to open at all; 4 and before won't open them.

I think many if not most of the features worked just fine circa 1993 or 1994. I remember my dad sending me some kind of PDF that wouldn't let him print it. I dug into the PDF spec and the "encryption" and protection bits were already there. I forget the details, but at the time I recall saying that I could have hacked up something to remove the security in a day or two, and couldn't he just use screen captures, which he then did. It's a good thing, in hindsight, that I didn't hack the tool together, or it would have been me instead of Skyarov...