import { FileOpener } from '@ionic-native/file-opener';

constructor(private fileOpener: FileOpener) { }

...

this.fileOpener.open('path/to/file.pdf', 'application/pdf')
  .then(() => console.log('File is opened'))
  . catch (e => console.log('Error opening file', e));

FileUriExposedException: file:///storage/emulated/0/test.txt exposed beyond app through Intent.getData()

Android APK installation restrictions
When opening the APK file for installation, the following restrictions apply:

In Android 8 + on, your application must have ACTION_INSTALL_PACKAGE permission. You can add it by adding it to your application config.xml file:
     <platform name= " android " >
        <config-file parent="/manifest" target="AndroidManifest.xml" xmlns:android="http://schemas.android.com/apk/res/android">
            <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
        </config-file>
    </platform> 
Before Android 7, you could only install APKs from the "external" partition. For example, you can install cordova.file.externalDataDirectory from it, but not cordova.file.dataDirectory from it. Android 7 + does not have this limitation.