A Study of Selexipag as Add-On Treatment to Standard of Care in Children With Pulmonary Arterial Hypertension

Inclusion Criteria: * Participants between greater than or equal to (\>=) 2 and less than (\<) 18 years of age weighing \>=9 kilogram (kg) at randomization * Pulmonary arterial hypertension (PAH) diagnosis confirmed by documented historical right heart catheterization (RHC) performed at any time before participant's screening * PAH (World Health Organization \[WHO\] Group 1), including participants with Down syndrome, of the following etiologies: Idiopathic PAH (IPAH); Heritable PAH (HPAH); PAH associated with congenital heart disease (PAH-associated with congenital heart disease \[aCHD\]) (PAH with coincidental CHD \[that is, a small atrial septal defect, ventricular septal defect, or patent ductus arteriosus that does not itself account for the development of elevated PVR\] and if approved by the BCAC) and Post-operative PAH (persisting / recurring/ developing \>=6 months after repair of CHD); Drug or toxin-induced; PAH associated with Human immunodeficiency virus (HIV) * WHO functional class (FC) II and III * Participants treated with at least 1 PAH-specific treatment, example, an Endothelin receptor antagonist (ERA) and/or a Phosphodiesterase type-5 (PDE-5) inhibitor/soluble guanylate cyclase stimulator, provided that the treatment dose(s) has been stable for at least 3 months prior to first dose of study intervention Exclusion Criteria: * PAH due to portal hypertension, schistosomiasis, pulmonary veno-occlusive disease, and/or pulmonary capillary hemangiomatosis * PAH…