Sources that include general collections of public presidential papers:
American Presidency Project
Site contains documents (1789 - present) related to the study of the American Presidency, including public papers, annual messages to Congress, inaugural addresses, radio addresses, acceptance speeches, presidential candidates debates, party platforms, elections data, and an audio/video archive.
Public Papers of the Presidents of the United States (FDsys) (1991-2009, from Bush I through Obama)
"The Public Papers of the Presidents is published by the Office of the Federal Register (OFR) and is the official publication of United States Presidents' public writings, addresses, and remarks.
Each Public Papers volume contains the papers and speeches of the President of the United States that were issued by the Office of the Press Secretary during the specified time period. The material is presented in chronological order, and the dates shown in the headings are the dates of the documents or events. In instances when the release date differs from the date of the document itself, that fact is shown in the textnote. Files are available in ASCII text and PDF formats." - GPO
*NOTE: A Library Catalog title search for "public papers of the presidents" will also retrieve links and records for papers of specific administrations (e.g., Truman, Kennedy, Nixon), both in print and online.