If you are suddenly having trouble saving documents to your SD card, please check to see if your SD card is corrupted.
This tutorial assumes you have an SD card mounted to Xodo already, see:

1. Tap the Overflow button to open the overflow menu from the toolbar

2. tap Save a Copy from the menu followed by Identical Copy from the sub menu

3. Select the overflow button in the dialog

4. Select SD Card

After that, select a folder of your choice to save the file to.