This study is an international, multicenter, randomized (1:1 oral treprostinil (UT-15C): placebo), double-blind, placebo-controlled study in subjects with Pulmonary Arterial Hypertension (PAH) who are currently receiving background oral monotherapy for at least 30 days at randomization for their PAH. Study visits will occur in four week intervals for the first 12 weeks, then every 12 weeks thereafter with key measures of efficacy being time to first clinical worsening event and the 6-minute walk test at Week 24. Study procedures include routine blood tests, medical history, physical exams, disease evaluation, and exercise tests. Patients who complete all required assessments will also be eligible to enter a long-term, open-label, extension study (TDE-PH-311).