It seems that you won't be able to use the BAK file on your Android device together with WhatsApp Messenger. According to the information found on the Internet, this kind of files can be opened on a Windows platform by using the Notepad. However, on your Android device, I suggest to head over to Android Informer's database, in order to search for an application that can work with this type of file.
If you can't find an application, just transfer the file to a PC, and open it from there.
