This study will compare the efficacy, safety, and tolerability of three doses of selonsertib (GS-4997) to placebo in adults with pulmonary arterial hypertension (PAH). The study will consist of a 24-week placebo-controlled treatment period and a long-term selonsertib treatment period. Participants completing the 24-week placebo-controlled period will be eligible to receive active treatment with selonsertib in the long-term treatment period.