viewer vs observer

viewer and observer both are nouns.

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

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

viewer: a close observer; someone who looks at something (such as an exhibition of some kind)
observer: a person who becomes aware (of things or events) through the senses

Other hypernyms of viewer include beholder, perceiver, percipient.

viewer (noun)	observer (noun)
an optical device for viewing photographic transparencies	a person who becomes aware (of things or events) through the senses
a close observer; someone who looks at something (such as an exhibition of some kind)	an expert who observes and comments on something