This study is researching an experimental drug called ALN-CFB. The study is focused on people with Paroxysmal Nocturnal Hemoglobinuria (PNH) who are currently taking a complement component C5 inhibitor ("C5-inhibitor") and continue to have anemia (low red blood cell count). The aim of the study is to see how tolerable ALN-CFB is compared to placebo. A placebo looks like the study drug but does not contain any drug. The study is looking at several other research questions, including: * What side effects may happen from taking ALN-CFB * How much ALN-CFB is in the blood at different times * How much Complement Factor B (CFB) protein levels in the blood are affected by ALN-CFB