To open a file that is stored on the external memory card in your device, you need to navigate to the location of the file, tap on it in order to open and choose from the list provided the Polaris Viewer. If you have set up a default application to open a certain file type, you can go to Settings > Applications > Manage Applications > All tab, tap on the default application and Clear Data / Clear Cache. After you did this, try to open the file again. The list of compatible applications that you can use should appear now and you will be able to chose Polaris Viewer.
