For 20 years after he left government, Kissinger succeeded in monopolizing access to these files, which he took with him in December 1976. He first sequestered these papers at the Rockefeller estate (in Pocantico Hills, New York) and only when reporters started asking questions and even filing Freedom of Information Act (FOIA) lawsuits, did he gift them as a closed collection to the Library of Congress -- Congress having conveniently exempted itself from the FOIA. In the 1990s, when State Department historians began preparing the Foreign Relations of the United States documentary volumes on the Nixon years, they asked for access to the collection, which Kissinger granted (with significant restrictions -- no photocopies, no note-taking), and in going through it, they found records that did not exist in the regular government files -- even though Kissinger had claimed he only took copies when he left office.
There was a happy ending to this story, but not until my own organization, the National Security Archive, threatened to bring legal action against the State Department and the National Archives for abdicating their duty under the records laws, did the government finally ask Kissinger to return copies of the records, which he did in 2001. Securocrats are still declassifying some of the juiciest items, like this one.