A witness is someone who can give an account of or tell what happened because they were there. If you are a witness to an event, you were there and can provide a first hand account of what occurred. Used as a verb, to witness something is to see it happening.
Ex: Because Jane was a witness to the incident at the bank, the manager asked her to stay and speak to the police about what she had seen.
Ex: I don‘t know what my insurance company is going to do about this disputed accident claim. It was so dark and stormy that night, and no witness exists.
To bear witness is to testify that you saw something or observed something to be true. You can bear witness, or testify in a court of law, to someone's character, or about something that occurred.
Ex: If you are willing, you can bear witness at my upcoming court date that the no parking sign was obscured by foliage and I had no way of seeing it.
Ex: Frank's best friend was asked to bear witness to his character in a job recommendation that he needed to fill out online.
When you use the word viewer, you are talking about someone who sees something, usually through a medium such as television or the Internet.
Ex: As the viewer can tell by my umbrella, the storm we are experiencing has high winds with even stronger gusts every few moments.
Ex: In just a few moments, we will ask our viewers at home to call in their pledges to help support this public television station and its educational programming.
The word viewer can also refer to a device that can be used to look at pictures or slides, especially one used in years gone by.
Ex: Before the days of radio and television, party¬goers might look at pictures of exotic places through a viewer that allowed a piece of film to be easily seen.
Ex: My brother and I used to fight over the viewer that allowed us to look at pictures of zoo animals that my father had taken on one of our vacations