The beauty of plain text schematics is that ASCII text is always recognisable - once you get past the fixed font criteria.
All the modern graphic variants require some form of GUI display to render them, and as history has shown time and time again these formats come and go and someone always tries to push another one onto the market, or as Microsoft is keen to to add features that make the file unusable on earlier versions.
.PDF is probably the most reliable in ensure the page is presented as intended, but even then the earliest readers would not be able to open a new PDF file
Plain ASCII - well you could probably squirt that to an ASR33 teleprinter and still be able to read it fine!
Dinosaur bones last quite some time in the ground!