A very good rule of thumb with data formats is to pick those that are no more complex than the data being represented, that are recoverable with simple tools and widely implemented. In general, if you have written a text document and it's not viewable and editable in a low-level text editor like Notepad (or Emacs, Vim, TextMate, BBEdit, gedit, kate, pico/nano etc.), you should probably take the time to convert it into a plain-text format - keep the rich format also. If you are backing up data in a format that's not widely understood, be sure to also keep backups of the software you use to open it and any registration keys - as you may find that a file made with version 2.x of a piece of software won't open the all new, singing and dancing version 5.x!
Plain text, HTML and non-bloated XML formats are all good bets (DocBook, TEI etc.).
The Portable Document Format standard created by Adobe has reached a point where it should be readable for posterity. It is now open enough that it should have the ability to be read long into the future. You can get Adobe Acrobat Reader here.
The TeX standard has been around since 1969. TeX documents are text based. It is widely used to prepare multi-thousand page documents for publication, as well as mathematical formula. LaTeX is an open implementation of this standard.