I'm only vaguely aware of the de-proprietary-ization of PDF; I remember at some point the market was flooded with print-to-PDF software, and commentary that this was happening because of something had somehow changed.
Adobe made the file format open source to spur adoption. It worked.
Make no mistake, there were a good number of PDFs floating around before that point. It was pretty much immediately popular, but they were kind of a hassle for everybody not running Windows. There were programs to display them like xpdf, but they weren't bug compatible and lived in a legal grey area. Everything got a lot better once Adobe released the rights on the format.
What happened?