viewer vs percipient

viewer and percipient both are nouns.

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

As nouns, percipient is a hypernym of viewer; that is, percipient 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)
percipient: a person who becomes aware (of things or events) through the senses

Other hypernyms of viewer include beholder, observer, perceiver.

viewer (noun)	percipient (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)

viewer (adjective)	percipient (adjective)
	characterized by ease and quickness in perceiving