signer vs attestor

signer and attestor both are nouns.

Word	Noun	Adjective	Verb	Adverb
signer	Yes	No	No	No
attestor	Yes	No	No	No

As nouns, attestor is a hyponym of signer; that is, attestor is a word with a more specific, narrower meaning than signer:

signer: someone who signs and is bound by a document
attestor: (law) a person who attests to the genuineness of a document or signature by adding their own signature

Other hyponyms of signer include cosignatory, cosigner, cosigner, abhorrer, endorser, indorser, attestant, attestator, witness.

signer (noun)	attestor (noun)
someone who signs and is bound by a document	(law) a person who attests to the genuineness of a document or signature by adding their own signature
someone who can use sign language to communicate