http://library.stmarytx.edu/acadlib/doc/guides/history/inst.htm#webpres

Presidential documents are classified PR in the Documents stacks, with a number following the PR designating the presidency. For example, publications from Jimmy Carter's presidency are classified PR 39.... An important recurring series within the Presidential publications, the Economic Report of the President, is classified PR xx.9: , where "xx" is the number of the presidency. Again, the Ecomic Reports issued while Carter was President are classified PR 39.9: (year).

Source: Blume Library, St. Mary's University - San Antonio, Texas