papers vs written material

papers and written material both are nouns.

Word	Noun	Adjective	Verb	Adverb
papers	Yes	No	No	No
written material	Yes	No	No	No

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

papers: writing that provides information (especially information of an official nature)
written material: the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)

Other hypernyms of papers include piece of writing, writing.

papers (noun)	written material (noun)
writing that provides information (especially information of an official nature)	the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)