Winners write the history books
There is no mention here of Novell Envoy (1995), which I had, as part of PerfectOffice, and used. A free downloadable viewer (which no one had) and the ability to package the viewer together with the .ENV file as an .EXE, which was quickly to become an unacceptable attachment in several systems as did a similar emission from OmniForm.
Perhaps none of these competitors threatened -- or will ever threaten? -- the economic success of Adobe, but they must have had some influence.