Twentyfive years ago, on July 1st, 1993, Addison Wesley has published the first Portable Document Format Reference Manual:
Initially the specification was only available as a printed book which had to be purchased. Much later the spec was made available as a free PDF version!
John Warnock, one of the Adobe founders, had described the basics of PDF already in his «Camelot» white paper. Then the new data format was called Interchange PostScript (IPS) and was meant to facilitate the paperless office.