On RSA, knowing ay of the factors used is enough to break the key. Thus, you brute force 256 bits RSA with the same number of tries that you brute force 128 bits AES.
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.
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.