In order to view remote documents from an HTTP URL, cleartext network traffic support is required. On Android 9.0 (API level 28) or higher, cleartext support is disabled by default and apps targeting Android 9.0 or higher will need to add the android:usesClearTextTraffic="true" flag in the AndroidManifest.xml file. If you are only working with HTTPS files, this flag is not required.

Example:

<?xml version="1.0" encoding="utf-8"?>
<manifest ...>

    <!-- Required permissions are added here -->
    <uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.RECORD_AUDIO" />

    <!-- Add multidex support and enable largeHeap.
         Also enable usesCleartextTraffic in application attributes if you are 
         working with HTTP files. If you are only working with HTTPS files, this is not required.-->
    <application 
        android:name="androidx.multidex.MultiDexApplication"
        android:largeHeap="true"
+       android:usesCleartextTraffic="true"
        ...>    
        ...
    </application>
</manifest>