Most often, this is a Corel WordPerfect document. But to be sure, ask the sender.

The options you have:
a. Buy and install Corel WordPerfect
b. See if your current word processor (MS Word? OO Writer?) opens the document either immediately or after installing the right converter.
c. If MS Word doesn't, find out if OO Writer does (sorry, I don't know, the last time I saw a WordPerfect document was in 1997).
d. Ask the sender to send it in a format you (and all Windows users) can read. Rtf seems fine, because it's understood by Wordpad also and that comes with Windows. And sorry again, I don't know about Mac OS and Linux.