What is a viewer?
A viewer is an application that can recognize and process a data file whose name is passed to it from another application. When you open a file from a file manager, the operating system passes the file’s name to the application currently associated with the type of that file. A true viewer will display, play, or otherwise process the passed file. An application that can display (for example) images is still not a viewer, unless it is programmed to accept and process a passed file name.