<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <!--PDF View for displaying our PDF-->
    <com.github.barteksc.pdfviewer.PDFView
        android:id="@+id/pdfView"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</RelativeLayout>

import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import com.github.barteksc.pdfviewer.PDFView;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends AppCompatActivity {

    // creating a variable for our Firebase Database.
    FirebaseDatabase firebaseDatabase;

    // creating a variable for our Database 
    // Reference for Firebase.
    DatabaseReference databaseReference;

    // creating a variable for our pdfview
    private PDFView pdfView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // initializing variable for pdf view.
        pdfView = findViewById(R.id.pdfView);

        // below line is used to get the instance 
        // of our Firebase database.
        firebaseDatabase = FirebaseDatabase.getInstance();

        // below line is used to get reference for our database.
        databaseReference = firebaseDatabase.getReference("url");

        // calling method to initialize
        // our PDF view.
        initializePDFView();
    }

    private void initializePDFView() {

        // calling add value event listener method 
        // for getting the values from database.
        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                // this method is call to get the realtime updates in the data.
                // this method is called when the data is changed in our Firebase console.
                // below line is for getting the data from snapshot of our database.
                String pdfUrl = snapshot.getValue(String.class);

                // after getting the value for our Pdf url we are 
                // passing that value to our RetrivePdfFromFirebase
                // class which will load our PDF file.
                new RetrivedPdffromFirebase().execute(pdfUrl);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                // calling on cancelled method when we receive
                // any error or we are not able to get the data.
                Toast.makeText(MainActivity.this, "Fail to get PDF url.", Toast.LENGTH_SHORT).show();
            }
        });
    }

    class RetrivedPdffromFirebase extends AsyncTask<String, Void, InputStream> {
        // we are calling async task and performing 
        // this task to load pdf in background.
        @Override
        protected InputStream doInBackground(String... strings) {
            // below line is for declaring
            // our input stream.
            InputStream pdfStream = null;
            try {
                // creating a new URL and passing 
                // our string in it.
                URL url = new URL(strings[0]);

                // creating a new http url connection and calling open
                // connection method to open http url connection.
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                if (httpURLConnection.getResponseCode() == 200) {
                    // if the connection is successful then 
                    // we are getting response code as 200.
                    // after the connection is successful 
                    // we are passing our pdf file from url
                    // in our pdfstream.
                    pdfStream = new BufferedInputStream(httpURLConnection.getInputStream());
                }

            } catch (IOException e) {
                // this method is 
                // called to handle errors.
                return null;
            }
            // returning our stream
            // of PDF file.
            return pdfStream;
        }

        @Override
        protected void onPostExecute(InputStream inputStream) {
            // after loading stream we are setting 
            // the pdf in your pdf view.
            pdfView.fromStream(inputStream).load();
        }
    }
}