Main objective: Evaluation of the pharmacodynamic effect of the combination of Sildenafil and Riociguat on blood pressure and other safety parameters in patients with symptomatic PAH Secondary objectives:To investigate the safety of the riociguat/sildenafil combination and changes in 6-minute walk test, World Health Organization (WHO) functional class, N terminal pro-brain natriuretic peptide, and variables obtained during right-heart catheterization after 12 weeks of treatment; pharmacokinetics of riociguat and sildenafil.