Notice that the problem in question is: 'is a brute-force attack against a 256-bit AES encrypted document feasible?'.
But only that: there is no cryptanalysis in that. So read with the statement in mind.
Edit: And notice that any 256-bit key encryption algorithm is as safe as AES under brute-force.
Or, in other words, when you create a 256 bits RSA key, it's guaranteed that one of the factors will have at most 128 bits. Thus, it's only as strong as a 128 bits AES against brute force.
But anyway, that's a moot point, because criptoanalysis exists, and there are better than brute force attacks against both algorithms.