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.