Background: NPAPI = Netscape plug-in API. Invented a long time ago as a neat way to let one app show content in another app. There are still all sorts of uses of it. Chrome, however, has phased it out -- NPAPI deprecation: developer guide - The Chromium Projects
In this although both Flash and Acrobat are Adobe products, they need a way to talk to each other, and so they both use NPAPI.