I was rejected once while trying to save a spreadsheet in .htm format and use it as an attachment. The message I got was:
<font color=blue>We encountered a problem. The reason reported was:
The files you may upload must have the following extension: .zip,.txt,.gif,.jpg,.jpeg,.bmp,.wav,.mp3,.mpeg,.mpg,.avi,.pdf,.png </font color=blue>.
Harv, as you noted, .pdf files are acceptable. Adobe .pdf files are probably the most readable for the most people, yet to generate one you need the full $ version of Adobe, not just the freeware version.