On your Library, select the file you want to download.
tap the kebab icon next to the filename and select Download Audio
tap on the Voice you want to use for offline reading
make sure you have a strong and stable Internet connection and you're in the active foreground. That means you have to wait while it's processing and your phone screen is not locked until importing is done
once the file is successfully downloaded with the offline voice, you will see a 'green arrow' next to the Voice
You can remove the file after you are done listening to it by clicking on the three dots next to the file name again and selecting 'Remove Audio'.
Note: I suggest trying out these steps before going off-grid. Also, it is recommended to remove audio from the app after you are done listening to it as the files take up memory in your device and too much memory consumption may slow down your phone depending on your device specifications.