Study ROR-PH-301 is a multicenter, randomized, double-blind, placebo-controlled study. Subjects who meet entry criteria will be randomly allocated 1:1 to receive ralinepag or placebo, in addition to their standard of care or PAH-specific background therapy, as applicable. The primary endpoint is the time (in days) from randomization to the first adjudicated protocol-defined clinical worsening event. All primary endpoint events will be adjudicated by an independent Clinical Event Committee (CEC) in a blinded fashion. Subjects who have a confirmed primary endpoint event adjudicated by the CEC at any time during the study and all subjects on treatment at the conclusion of the study (after the target number of events is achieved) will have the option to enroll in an open-label extension (OLE) study.