A Study to Assess Whether Macitentan Delays Disease Progression in Children With Pulmonary Arterial Hypertension (PAH)

Key Inclusion Criteria: * Signed informed consent by the parent(s) or legally designated representative and assent from developmentally capable children prior to initiation of any study-mandated procedure * Males or females between greater than or equal to (\>=) 1 month and less than (\<) 18 years of age * Participants with body weight \>= 3.5 kilograms (kg) at randomization * Pulmonary arterial hypertension (PAH) diagnosis confirmed by historical RHC (mPAP greater than or equal to \[\>=\] 25 millimeters of mercury \[mmHg\], and Pulmonary artery wedge pressure \[PAWP\] less than or equal to \[\<=\] 15 mmHg, and Pulmonary vascular resistance index \[PVRi\] greater than \[\>\] 3 WU × m2), where in the absence of pulmonary vein obstruction and/or significant lung disease PAWP can be replaced by Left atrium pressure \[LAP\] or Left ventricular end diastolic pressure \[LVEDP\] (in absence of mitral stenosis) assessed by heart catheterization * PAH belonging to the Nice 2013 Updated Classification Group 1 (including participants with Down Syndrome) and of following etiologies: idiopathic PAH; heritable PAH; PAH associated with congenital heart disease (CHD); Drug or toxin induced PAH; PAH associated with HIV; PAH associated with connective tissue diseases (PAH-aCTD); and World health organization (WHO) Functional class I to III * Females of childbearing potential must have a negative pregnancy test at Screening and at Baseline, and must agree to undertake monthly pregnancy tests, and to use a reliable method of contraception (if sexually active) up to the end of study (EOS) Key Exclusion Criteria: * Participants with PAH due to portal hypertension, schistosomiasis, or with pulmonary veno-occlusive disease and/or pulmonary capillary hemangiomatosis, and persistent pulmonary hypertension of the newborn * Participants with PAH associated with Eisenmenger syndrome, or with moderate to large left-to-right shunts * Participants receiving a combination of \> 2 PAH-specific treatments at randomization. * Treatment with intravenous (IV) or subcutaneous (SC) prostanoids within 4 weeks before randomization, unless given for vasoreactivity testing * Hemoglobin or hematocrit \<75 percent (%) of the lower limit of normal range * Serum Aspartate aminotransferase (AST) and/or Alanine aminotransferase (ALT) greater than (\>) 3 times the upper limit of normal range * Pregnancy (including family planning) or breastfeeding. * Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol * Severe hepatic impairment, for example Child-Pugh Class C * Clinical signs of hypotension which in the investigator's judgment would preclude initiation of a PAH-specific therapy * Severe renal insufficiency (estimated creatinine clearance \<30 mL/min or serum creatinine \>221 micro-moles per liter \[micro-mol/L\]) * Participants with known diagnosis of bronchopulmonary dysplasia