Key Inclusion Criteria  PAH belonging to one of the subgroups:  I/HPAH, PAH-CTD, PAH due to drugs and/or toxins/chemicals (having been in the care of the investigator for at least one year with no relapses of drug or toxin/chemical abuse), HIV associated or PAH due to repaired congenital heart disease (at least 1 year since repair) World Health Organization (WHO) Functional Class II, III or IV symptoms Stable concomitant background therapy of at least two PAH approved medications Able to walk a distance of at least 100 m but no more than 475 m during the Screening 6-minute walk tests.  Key Exclusion Criteria  Pulmonary hypertension (PH) belonging to Groups 2 to 5 A history of left-sided heart disease Pregnant or breast-feeding females  Additional criteria may apply, per protocol