【Paper】 ,  【instrument】 ,  【document】  all mean a writing (often typed, sometimes printed) that is of value to its owner or to others who come after him as a source of information or proof of a right, contention, or claim.

【Paper】 is the most general term, applicable to such writing (as a letter, deed, certificate, or writ) filed away for future use or reference.

【Instrument】 is a legal term applicable to a 【paper】 (as a deed, a writ, a will, or a contract) that is made and executed according to the terms of the law, as concrete evidence of some legally defined action (as a transfer of property, the enforcement of a judgment, one′s decisions as to who shall inherit one′s property, or the terms of an agreement).

【Document】  (see also  【DOCUMENT】 1 ) applies to a legal 【instrument】 or to an original or authentic copy of a letter, a record, or other 【paper】 that may be used as a source of information, evidence, or proof.