How to write a script to assign adobe reader as the default application for opening PDFs on Android?
There is a deprecated api which allows to set preferred activities for specific intents. So you could for example set the adobe reader as the preferred activity to start when "open pdf" intent is sent

However as it states this api is deprecated so it could be removed in future versions of android

http://developer.android.com/reference/android/content/pm/PackageManager.html#addPreferredActivity(android.content.IntentFilter, int, android.content.ComponentName[], android.content.ComponentName)