You can't prompt them to upgrade to the latest version of Reader because you can't know what's the latest version of Reader. What you can do, though, is check if they have a version earlier than 11, for example, and then prompt them to upgrade.
Something like this:
if (app.viewerType=="Reader" && app.viewerVersion<11) {
if (app.alert("It is recommended that you upgrade your version of Reader to the latest one. Do you wish to do so now?",2,2)==4) {
app.launchURL("http://get.adobe.com/reader/");
}
}
.
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