When you said Red Hat Linux 5. Can I confirm you don't mean Red Hat Enterprise Linux 5 (Red Hat 5 is very old).
I was able to find the answer to this on wikipedia...
Looking under the GTK section there are three things that should be able to display these files for you.
FBReader appears to come in Fedora already, but I can't see it on CentOS so it's most probable it is not in RHEL