A Clinical Study of to Confirm the Doses of Selexipag in Children With Pulmonary Arterial Hypertension

Inclusion Criteria: * Signed and dated informed consent by the parent(s) or Legally authorized representative(s) AND assent from developmentally capable children * Males or females between greater than or equal to (\>=) 2 and less than (\<) 18 years of age with weight \>= 9 kilograms (kg) * Pulmonary arterial hypertension (PAH) diagnosis confirmed by documented historical right heart catheterization (RHC) performed at any time before participant's enrollment * PAH with one of the following etiologies: * idiopathic (iPAH), * heritable (hPAH), * associated with congenital heart disease (CHD): PAH with co-incidental CHD; post-operative PAH (persisting/ recurring/ developing \>= 6 months after repair of CHD) * Drug or toxin-induced * PAH associated with HIV * PAH associated with connective tissue disease * Word Health Organization functional class (WHO FC) II to III * Participants treated with an endothelin receptor antagonist (ERA) and/or a phosphodiesterase type 5 (PDE-5) inhibitor provided that the treatment dose(s) has been stable for at least 3 months prior to enrollment, or participants who are not candidates for these therapies * Females of childbearing potential must have a negative pregnancy test at Screening and at Enrollment, and must agree to undertake monthly pregnancy tests, and to use a reliable method of contraception (if sexually active) from screening up to study drug discontinuation plus 30 days (EOS) Key Exclusion Criteria: * Participants with PAH due to portal hypertension, schistosomiasis, pulmonary veno-occlusive disease (PVOD) and/or pulmonary capillary hemangiomatosis * Participants with PAH associated with Eisenmenger syndrome * Participants with moderate to large left-to-right shunts * Participants with cyanotic congenital cardiac lesions such as transposition of the great arteries, truncus arteriosus, univentricular heart or pulmonary atresia with ventricular septal defect, as well as Participants with Fontan-palliation * Participants with pulmonary hypertension due to lung disease * Previous treatment with Uptravi (selexipag) within 2 weeks prior to enrollment * Participants having received prostacyclin (epoprostenol) or prostacyclin analogs (that is, treprostinil, iloprost, beraprost) within 2 months prior to enrollment or are scheduled to receive any of these compounds during the trial * Treatment with another investigational drug within 4 weeks prior to enrollment * History, or current suspicion of intussusception or ileus or gastrointestinal obstruction as per investigator's judgment * Uncontrolled thyroid disease as per investigator judgment * Hemoglobin or hematocrit \< 75 percentage (%) of the lower limit of normal range * Known severe or moderate hepatic impairment * Clinical signs of hypotension that in the investigator's judgment would preclude initiation of a PAH-specific therapy * Participants with severe renal insufficiency * Known hypersensitivity to the investigational treatment or to any of the excipients of the drug formulations