living will vs legal document

living will and legal document both are nouns.

Word	Noun	Adjective	Verb	Adverb
living will	Yes	No	No	No
legal document	Yes	No	No	No

As nouns, legal document is a hypernym of living will; that is, legal document is a word with a broader meaning than living will:

living will: a document written by someone still legally capable requesting that he should be allowed to die if subsequently severely disabled or suffering terminal illness
legal document: (law) a document that states some contractual relationship or grants some right

Other hypernyms of living will include instrument, legal instrument, official document.

living will (noun)	legal document (noun)
a document written by someone still legally capable requesting that he should be allowed to die if subsequently severely disabled or suffering terminal illness	(law) a document that states some contractual relationship or grants some right