 This study is designed to evaluate the safety, tolerability, pharmacodynamics and pharmacokinetics of riociguat at age-, sex- and body-weight-adjusted doses of 0.5 mg, 1.0 mg, 1.5 mg, 2.0 mg and 2.5 mg TID in children from 6 to less than 18 years with pulmonary arterial hypertension PAH group 1. The study consists of two phases: titration phase up to 8 weeks and a maintenance phase up to 16 weeks. 