Android 4.4 KitKat imposed restrictions on applications “writing” to External Storage cards.
For Documents To Go, this means that with this version of Android, you are not able to save files on to the SD card directly from the app. Instead, you need to save files onto your internal memory card.
In previous versions of the Android OS, this restriction did not exist. Therefore, Docs To Go was able to allow you to save files where you chose.
Also, in version 5.0 and above of Android this limitation was removed.
If you are concerned about filling up your internal memory, AND you are unable to upgrade your Android phone, you can choose to save files to one of the many cloud services that Docs To Go Premium edition supports. These cloud services are Google Drive, Dropbox, Box and OneDrive.