According to Kerckhoffs' principle, the attacker knows the PRNG and decryption algorithm.Originally Posted byMario F.

What you described is a possible use of a PRNG to implement a stream cipher. This can be secure with the use of a cryptographically secure PRNG, but it does not have the property of being unbreakable like a one time pad. By observing sufficient ciphertext, it is theoretically possible to determine the state of the PRNG, and thus break the encryption.