The purpose of simple formats like txt or html is to protect from
   viruses and trojans. The more you can understand and rebuild yourself
   about your system, the fewer gaping security holes. Most of these huge
   holes are in graphical setups like desktop environments or web
   browsers, and proprietary products or formats like those of adobe or
   microsoft.
   Most text-based people nowadays are probably experienced with viewing a
   pdf in text mode. Academic papers, for example, are distributed this
   way. It can be a little frustrating if the content is scanned without
   OCR.
   The PDF books are great and inspiring. I would add a more safe or
   accessible format only when and if capacity allows.