Of course you can set the target operating system to associate the pdf extension with a web browser but I think you're probably looking for the form itself to be able to control that regardless of operating system.
If so, try putting this in the top level subform's docReady event:
if (!event.target.external){
cURL=event.target.path;
cURL=cURL.replace(/\/(\w)\//,"$1:/");
app.launchURL(cURL,true);
event.target.closeDoc();
}
Adobe will show you a warning to Cancel or Allow but that can't be supressed for security reasons.
Kyle