A Prospective, Randomized, International, Multicenter, Double-arm, Controlled, Open-label Study of Riociguat in Patients With Pulmonary Arterial Hypertension (PAH) Who Are on a Stable Dose of Phosphodiesterase-5 Inhibitors (PDE-5i) With or Without Endothelin Receptor Antagonist (ERA), But Not at Treatment Goal

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