There is nothing wrong with reading du’aa’s from a piece of paper if one has not memorized them, or writing the du’aa’s on a piece of paper and reading them at times when it is liked to say du’aa’, such as the end of the night, or during the night, or at other times. But if it is possible to memorize them and recite them from heart and with proper humility and focus, that is better. 

With regard to prayer, it is better to recite them from memory, and the du’aa’s should be short and brief. If they are read from a piece of paper during the tashahhud, for example, or between the two prostrations, there is nothing wring with that, but if you memorize the du’aa’, that is closer to proper focus. And Allaah is the Source of strength. End quote.