<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 
    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"
    android:gravity="center"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <!--We will click on it to view pdf-->
    <Button
        android:id="@+id/view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:background="@color/black"
        android:padding="10dp"
        android:text="Click here to View pdf "
        android:textSize="10dp" />

</LinearLayout>

import android.content.DialogInterface;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

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

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;

public class MainActivity extends AppCompatActivity {

    Button view;
    DatabaseReference database;
    String message;

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

        // Initialising the reference to database
        database = FirebaseDatabase.getInstance().getReference().child("pdf");
        database.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                // getting a DataSnapshot for the location at the specified
                // relative path and getting in the link variable
                message = dataSnapshot.getValue(String.class);
            }

            // this will called when any problem
            // occurs in getting data
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                // we are showing that error message in toast
                Toast.makeText(MainActivity.this, "Error Loading Pdf", Toast.LENGTH_SHORT).show();
            }
        });
        // After clicking here alert box will come
        view.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(final View v) {
                CharSequence options[] = new CharSequence[]{
                        "Download",
                        "View",
                        "Cancel"
                };
                AlertDialog.Builder builder = new AlertDialog.Builder(v.getContext());
                builder.setTitle("Choose One");
                builder.setItems(options, new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        // we will be downloading the pdf
                        if (which == 0) {
                            Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(message));
                            startActivity(intent);
                        }
                        // We will view the pdf
                        if (which == 1) {
                            Intent intent = new Intent(v.getContext(), ViewPdfActivity.class);
                            intent.putExtra("url", message);
                            startActivity(intent);
                        }
                    }
                });
                builder.show();
            }
        });
    }
}

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

    <com.github.barteksc.pdfviewer.PDFView
        android:id="@+id/abc"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

import androidx.appcompat.app.AppCompatActivity;

import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Bundle;

import com.github.barteksc.pdfviewer.PDFView;

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

public class ViewPdfActivity extends AppCompatActivity {

    String urls;
    PDFView pdfView;
    ProgressDialog dialog;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_view_pdf);
        pdfView = findViewById(R.id.abc);

        // Firstly we are showing the progress 
        // dialog when we are loading the pdf
        dialog = new ProgressDialog(this);
        dialog.setMessage("Loading..");
        dialog.show();

        // getting url of pdf using getItentExtra
        urls = getIntent().getStringExtra("url");
        new RetrivePdfStream().execute(urls);
    }

    // Retrieving the pdf file using url
    class RetrivePdfStream extends AsyncTask<String, Void, InputStream> {

        @Override
        protected InputStream doInBackground(String... strings) {
            InputStream inputStream = null;
            try {

                // adding url
                URL url = new URL(strings[0]);
                HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();

                // if url connection response code is 200 means ok the execute
                if (urlConnection.getResponseCode() == 200) {
                    inputStream = new BufferedInputStream(urlConnection.getInputStream());
                }
            }
            // if error return null
            catch (IOException e) {
                return null;
            }
            return inputStream;
        }

        @Override
        // Here load the pdf and dismiss the dialog box
        protected void onPostExecute(InputStream inputStream) {
            pdfView.fromStream(inputStream).load();
            dialog.dismiss();
        }
    }
}