In order to use WhatsApp to PDF, you need to have the WhatsApp installed application on your device. Also, you need to have a rooted device, otherwise the application won't work. Try to open another file. If the same error occurs, try to go to the menu of your device and proceed to Settings > Applications > Manage application > All tab, select WhatsApp to PDF and tap on Clear Cache and Clear Data. After you did this, try again to open the file. If the problem persists, contact the developer on the following mail address for further assistance: firstname.lastname@example.org.