NaOH is a base. Therefore, the pH is initially very high and above 7. If you add HNO3, which is a strong acid and has a pH less than 7, you will lower the pH of the solution. In fact, at the equivalent curve, you will have a pH of 7 because you are mixing a strong acid and strong base. Depending on how much HNO3 is added, the pH will continue to go down.