Proof of it being public domain or not is a simple test, who wrote it and have they been dead for a certain amount of time. A long freaking time, most likely this is not the case. If the author put it into the public domain specifically (and it has to say so on it) it ain't PD until something like 70 years after the creators death. This isn't public domain. The Principia Discordia is public domain it says so right on it (All Rites Reversed)