In Google Nexus 7, an App can be specified to be launched by default for some actions. For example if you have got Kindle, Adobe and Document Viewer on your Google Nexus then you can use any one of the app to read a PDF file.
Associating an App with action
When you are reading a PDF file for the first time on Google Nexus, you will notice the following screen asking to Complete action using which App (Assuming these apps already installed on the device).
You can specify whether you want to use Always or only selected app for this action
The document will now be opened in the selected App.
Removing the App associated with any action
For example if you have specified Kindle as the app to be used always to read a PDF file and now you want to remove the action then you can follow the below mentioned steps.
Step 1: Launch the Settings screen on Google Nexus.
Step 2: In the Settings screen, tap the Apps option under Device section.
Step 3: Tap Amazon Kindle App in the Apps screen.
Step 4: Now tap the Clear defaults button to remove all actions for which this app will be launched by default.