As defined in Wikipedia, “Link rot” is an informal term for the process by which hyperlinks (either on individual websites or the Internet in general) point to web pages, servers or other resources that have become permanently unavailable. This has become a serious problem in legal opinions and decisions. A consortium of law libraries is taking action by creating Perma.cc.
The USA National Archives and Records Administration (NARA) has released a bulletin on January 31, 2014 detailing guidance on the use of PDF for electronic archival records.