For devices with Android KitKat, apps no longer have permission to write to an external SD card other than the app specific folder. If you are interested in the topic, here's what happened to external storage in KitKat. Therefore, we provide the option to save a copy to internal storage when files are opened from your external SD card. A possible workaround is using cloud storage, such as Dropbox, Google Drive or OneDrive, both of which are integrated with Xodo.

For devices above Android KitKat, such as Lollipop or Marshmallow, please follow this guide on opening and editing SD card files within Xodo: Accessing files on an external SD card