The purpose of this study is to find out if L606 is safe and if it helps people with high blood pressure in the lungs (pulmonary hypertension) caused by interstitial lung disease (PH-ILD, WHO Group 3). One of the main ways the study will check this is by seeing if people can walk further in six minutes (called the "six-minute walk test"). Another important (secondary) goal is to see how long it takes for PH-ILD to get worse while people are taking L606. One way the study will measure this is by looking at the distance people walk at different times in the study on different doses of L606. Throughout the study, doctors will also closely monitor for any side effects people have to make sure L606 is safe. People who decide to join this study will be randomly (like tossing a coin) placed into one of two groups: one group will get L606 and the other group will get a placebo, which will look like L606 but will not contain any active medicine. At the end of the initial blinded portion of the study, study participants may have the opportunity to enroll in an open-label extension portion of the study where all participants will receive L606.

Inclusion Criteria:

• Participant is between 18 years to 80 years old.
• Has a confirmed diagnosis of ILD based on High Resolution Computed Tomography (HRCT) chest imaging.
• Evidence of PH as demonstrated from Right Heart Catheterization (RHC) with the following hemodynamic parameters:
• Pulmonary vascular resistance (PVR) ≥ 320 dynes.sec/cm5 (i.e. 4 Woods Units (WU)), and
• Mean Pulmonary Artery Pressure (mPAP) of ≥ 25 mmHg, and
• - Pulmonary capillary wedge pressure (PCWP) or Left-Ventricular End Diastolic Pressure (LVEDP) of ≤ 15 mmHg.
• FEV1/FVC (ratio) > 0.70.
• 6-minute walk distance ≥ 150 meters

Exclusion Criteria:

• PH in the updated WHO Classification Groups 1, 2, 4, or 5.
• Has evidence of clinically significant left-sided heart disease as defined by echocardiography.
• Participants with history of persistent/permanent or uncontrolled atrial fibrillation.
• Participants with severe obstructive sleep apnea.
• Has experienced exacerbation of underlying lung disease or active respiratory infection requiring antibiotics.
• Initiation of pulmonary rehabilitation.

Note: Other protocol defined inclusion/exclusion criteria may apply.