tax return vs official document

tax return and official document both are nouns.

Word	Noun	Adjective	Verb	Adverb
tax return	Yes	No	No	No
official document	Yes	No	No	No

As nouns, official document is a hypernym of tax return; that is, official document is a word with a broader meaning than tax return:

tax return: document giving the tax collector information about the taxpayer's tax liability
official document: (law) a document that states some contractual relationship or grants some right

Other hypernyms of tax return include instrument, legal document, legal instrument.

tax return (noun)	official document (noun)
document giving the tax collector information about the taxpayer's tax liability	(law) a document that states some contractual relationship or grants some right