This is a Phase 3, randomized, double-blind, placebo-controlled, multinational, efficacy and safety study of subjects with PPF (Progressive Pulmonary Fibrosis) treated with inhaled treprostinil over a 52-week period. This study is investigating whether a drug called inhaled treprostinil (brand name Tyvaso®) works to help people with PPF improve their lung function tests. The purpose of this research study is to see how well inhaled treprostinil works in participants with PPF and to gather information on how safe it is. This study will look at changes in your breathing tests, also called lung function tests. About 698 people will participate in this study from about 150 medical centers. Your participation in this study is voluntary and will last approximately 58 weeks. This time includes a Screening Period that could last up to approximately 6 weeks plus a 52-week Treatment Period.