voucher vs written document

voucher and written document both are nouns.

Word	Noun	Adjective	Verb	Adverb
voucher	Yes	No	No	No
written document	Yes	No	No	No

As nouns, written document is a hypernym of voucher; that is, written document is a word with a broader meaning than voucher:

voucher: a document that serves as evidence of some expenditure
written document: writing that provides information (especially information of an official nature)

Other hypernyms of voucher include document, papers.

voucher (noun)	written document (noun)
a negotiable certificate that can be detached and redeemed as needed	writing that provides information (especially information of an official nature)
a document that serves as evidence of some expenditure
someone who vouches for another or for the correctness of a statement