Xodo offers the ability to utilize the system file picker provided by the OS.
Android:
On an Android device, from the Home page, you can select "+" > "Open file" to see the option of System Fils:
iOS:
On an iPhone or iPad, from the Home page, you can select "+" > "Open file" to see the option of System Fils:
Note that on iPhone or iPad, the file sources are controlled by the default Apple Files app. To add/remove sources, please use the Apple Files app.