For details see
http://www.wotsit.org/default.asp
RTF was a MS format for document interchange between different word
processor programs. It uses a standard of items most WP programs support
such as different fonts, font size, some paragraph formating etc.
HTML is a format for internet explorers to read and is designed to provide
rich documents over an internet connection. It has evolved as bandwidth has
increased but originally provided fancy web pages using just text with
formating commands imbeded in the text so more processing was done by the
computer for color, formating of text which was done by the recieving
computer saving amount of information that had to be transmited over the
remote connection. (Very simplified explaination)
PDF is a Adobe Propriorty formate designed to transmit fully formated
documents from one computer to another regardless of the platform or OS
using a standardized reader provided by Adobe. Originally it wasn't
designed for the reciever to need to do more than read the document and
print it without losing the format/fonts as they were embeded in the file.
Again, Simplified much.