When I open a file in SDCARD the app says: Documento is read-only. Save a duplicate file to edit? (This question is in Portuguese, but it is the same idea). Why does the app not open the file directly, without copying it to internal storage?
May I know the version of OS you are working with? If it is Android 4.4 or above then the SD Card is protected storage. Where you have installed the app? Is it on Internal Storage or SD Card. Also tell me the device you are using?