Tools > Options > Unprotect Document
If it's passworded you "probably" won't be able to do what you're trying to do.
You could try highlighting the word you want to underline and then pressing Ctrl and U together, that may still allow you to underline the words that you want to.
Ctrl+U =
Underline
Ctrl+I =
Italics
Ctrl+B =
Bold
(
Incidently, the same shortcut key combinations work here on CF as well 
)