Collocations for confidential - adj.

be
remain
consider sth as, hold sth (AmE), keep sth, treat sth as
The affair must be kept confidential.
Information about prices is to be treated as confidential.

highly
absolutely, completely, strictly, totally
The findings are strictly confidential.

commercially (BrE)
commercially confidential data

Confidential is used with these nouns:
advice, correspondence, document, enquiry, file, information, letter, material, memo, memorandum, nature, record, report, source