android studio download pdf file and save it in sharedpreferences

I have problem when trying to download PDF file and save it in internal storage or Shared-preferences and read it. Is it possible to download it in internal storage? I have followed several tutorials from internet but it doesn't work. could someone guide me how to do it? Thanks a lot.

package com.icul.downloadinternal;

import android.content.SharedPreferences;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.preference.PreferenceManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Base64;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

import java.io.IOException;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class MainActivity extends AppCompatActivity {

    Button download, read;
    ImageView pdf;
    String url = "https://www.cdc.gov/eval/guide/cdcevalmanual.pdf";

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

        download = (Button) findViewById(R.id.button);
        read = (Button) findViewById(R.id.button2);
        pdf = (ImageView) findViewById(R.id.image);

        download.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    downloadFileSync(url);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

        read.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                SharedPreferences shre = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
                String previouslyEncodedImage = shre.getString("pdf_data", "");

                if( !previouslyEncodedImage.equalsIgnoreCase("") ){
                    byte[] b = Base64.decode(previouslyEncodedImage, Base64.DEFAULT);
                    Bitmap bitmap = BitmapFactory.decodeByteArray(b, 0 , b.length);
                    pdf.setImageBitmap(bitmap);
                }
                else {
                    Toast.makeText(getApplicationContext(),"No File", Toast.LENGTH_LONG).show();
                }
            }
        });

    }
    public void downloadFileSync(String downloadUrl) throws Exception {

        OkHttpClient client = new OkHttpClient();
        Request request = new Request.Builder().url(downloadUrl).build();
        Response response = client.newCall(request).execute();
        if (!response.isSuccessful()) {
            throw new IOException("Failed to download file: " + response);
        }

        String encodedImage = Base64.encodeToString(response.body().bytes(), Base64.DEFAULT);
        SharedPreferences shre = PreferenceManager.getDefaultSharedPreferences(this);
        SharedPreferences.Editor edit = shre.edit();
        edit.putString("pdf_data", encodedImage);
        edit.commit();
    }
}