At least if you can refer to ePAPR right now, it's there for download
as a book-type spec for the basic elements, the same way you could with
OF, and the OF bindings were traditionally published in Postscript (and
written in tex? lord knows..

Many, or perhaps even all, of them were written in Frame Maker, which 
had relatively good support for handling large documents compared to its 
then competitors.  The ANS Forth standard, which was being developed at 
the same time, was written in MS Word.

PDF hadn't really taken off at the time.