eyewitness vs viewer

eyewitness and viewer both are nouns.

Word	Noun	Adjective	Verb	Adverb
eyewitness	Yes	No	Yes	No
viewer	Yes	No	No	No

As nouns, viewer is a hypernym of eyewitness; that is, viewer is a word with a broader meaning than eyewitness:

eyewitness: a spectator who can describe what happened
viewer: a close observer; someone who looks at something (such as an exhibition of some kind)

Other hypernyms of eyewitness include looker, spectator, watcher, witness.

eyewitness (noun)	viewer (noun)
a spectator who can describe what happened	an optical device for viewing photographic transparencies
	a close observer; someone who looks at something (such as an exhibition of some kind)

eyewitness (verb)	viewer (verb)
be present at an event and see it with one's own eyes