There are four categories of papers that are considered "presidential papers" 1.They are:
Public/published papers are proclamations, executive orders, presidential messages, speeches and statements. These are funnelled into established series (e.g. the Weekly Compilation of Presidential Documents)
Personal papers are those not created in the course of duty, such as personal diaries, journals, family momentos.
Executively Controlled documents are privileged from disclosure to congress or the courts because the incorporate national secrets, classified or legal information, or policy making records (e.g. the Nixon tapes), or National Security Directives. According to a 1992 GAO report, of the 1042 NSDs issued since 1961, only about 247 have been declassified. GAO tried to study these, but could only get a hold of 5. 2
Official documents are working documents generated in the course of daily presidential activitied. Until 1978 all official presidential records were considered private property. Now law mandates that they pass into custody of the Archivist of the United States. President Nixon caused the change in this law when he temporarily deposited his papers at the National Archives. Congress seized the Nixon papers by passing a law giving the government control over them.
1- Hirshon, Arnold, "The Scope, Accessability and History of Presidential Papers," Government Publications Review, v. 1:4, p.363-390.
2- United States. General Accounting Office, Report to the Chairman, Legislation and National Security Committee, Committee on Government Operations, House of Representatives. National Security: The Use of Presidential Directives to Make and Implement U.S. Policy. GAO/NSIAD-92-72, January 1992.