The Guardian uses a special format that is easy to read in emails. A short SMS message could provide everything necessary to improvise a lesson. (For a spoof on minimalist teaching see "The Ten Rules") In some environments, even in today's technologically sophisticated world, computers are unavailable or too much of a hassle to use.
Each file format can make different aspects of using a lesson easier. PDF files make printing out worksheets easier. An interactive self-correcting online elearning activity can be used at anytime without a teacher. Flash makes certain features of these activities easier like drag and drop. HTML pages are easy to read online.
There is no reason a program can't be written to reformat lessons in several different convenient formats.