It seems like Windows now disallows editing of default app if the store app does not explicitly define the supported file type. Comics++ by default associates with cbr/cbr7/cbz and not pdf because people usually use a different pdf reader so we didn't want to intrude on people's pdf settings.
You can probably still do it through some third party app, search "default programs editor"
You can probably still do it through some third party app, search "default programs editor"