Thanks for the reply. I finally got Plain Text to work. It needed to be told to access the entire Drop Box folder, not just it's own. That solves the one problem for .txt files.
The other two apps can edit other document files, but I haven't found a way to get them to save edited files to a Drop Box folder. I tried CloudOn and it seems to be an online app only. I need something that will allow me to edit files stored in Drop Box when offline and then have them sync up when I get back on line.