Thanks for the word hendiadys; I know (intimately) of the practice, but never had a word for it. My own certificate of appointment to my current (lawyer) job says that I am "authorized and empowered" to "execute and fulfill" my duties and "to have and to hold" my office, with all of its "powers, privileges, and emoluments."Legalese (lawyer's language) often includes what I remember as hendiadys from school days: you use a pair of words with the same meaning, nouns or adjectives, connected with "and", to make the meaning stronger. In their case, I think they use it to minimize the possibility that different nuances in the meanings are not covered.
I think it is typical in contracts, e.g., license agreements for software: "In any and every possible case you are bound to this EULA and you owe your soul to Microsoft".
I wonder if the meaning of the Greek phrase is something like" this certificate is a legally valid document"?