Android 10 compatibility

[KopfKrieg]

On an Android 10 device a warning message appears. It's rough translation reads as follows:

This app was developed for an older Android version and may not work properly. Check if updates are available or contact the developer.

Seems like an update is necessary in the future, not sure why though.

I'm running Document Viewer v2.8.1 from F-Droid.

Here's a picture (text is German):