There are two parts to this study. In Part 1 (week 0 to week 16), the objectives are to determine the safety, tolerability, efficacy, and exploratory objectives of two different doses of iNO delivered by a pulsed delivery device in symptomatic subjects with PAH who remain symptomatic due to PAH on approved PAH monotherapy or combination approved PAH therapy. In Part 2 (week 17 to end of study Part 2 [EOS Part 2]), the objective is to compile data on the long-term effects of iNO on safety, tolerability, clinical and hemodynamic measures.