I prefer .txt or .doc for the simple fact that I can read them while mobile on my phone.

Please have them in .txt so they can display in the webpage.

What about .pdf files? I've noticed that some stories are too long to get opened whole as .txt by the FA. I can upload txt files to scraps so the gallery won't have doubles.

I guess it comes down to personal preference really.
Personally I use .txt (A Text File, usually from Nopepad or something similar). This allows the text to be outright displayed on the webpage. The only downside to this is that you can't use text formatting such as Bold or Italic.