Adobe does have a secruity feature built in that keeps you from opening files or folders on the file system, but there is a way around this. First on your form you would put something like this on the click event of a button for your link:
var myPath = "o|/Network Folder/FileName";
openNetworkFolder(myPath);
Next you need to set up a JavaScript file that you will save as a trusted function for Reader. This js file should have the following code:
openNetworkFolder = app.trustedFunction(function (sURL) {
app.beginPriv();
try {
var nDoc = app.launchURL("file:///" + sURL, true);
} catch (err) {
app.alert('Error: ' + err);
}
app.endPriv();
});
This js file will need to be saved on each users local machine in their javascripts folder for reader. Also if you are pointing to a shared network folder, each user will need to have the same drive mapped for this to work properly for each user. This workaround allows you to bypass the built in Adobe security without having to sign the document.