hello vs how-do-you-do

hello and how-do-you-do both are nouns.

Word	Noun	Adjective	Verb	Adverb
hello	Yes	No	No	No
how-do-you-do	Yes	No	No	No

As nouns, hello and how-do-you-do are synonyms defined as:

Other synonyms of hello include hi, howdy, hullo.

hello (noun)	how-do-you-do (noun)
an expression of greeting	an expression of greeting
	an awkward situation