Fatal error:An error occured while executing doInBackground() - android-asynctask

Question

I am new to android and i am getting this error.Maths1 code:
package com.materialdesign.geniusmathsandphysics;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.net.ConnectivityManager;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;
import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPFile;
import org.apache.commons.net.ftp.FTPReply;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
public class Maths1 extends Activity{
MainActivity async;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.maths1);
async=new MainActivity();
Button m15,d15;
m15 = (Button) findViewById(R.id.but_m1may15);
d15 = (Button) findViewById(R.id.but_m1dec15);
m15.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
File pdf = new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/GeniusPhysicsandMaths/" + "M1_May_15.pdf");
if (pdf.exists()) {
//file= new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/pdf/Area.pdf");
Intent i=new Intent(Maths1.this,PdfViewer.class);
i.putExtra("FileName",pdf);
startActivity(i);
} else {
if (isNetworkConnected() == true) {
Toast.makeText(Maths1.this, "Starting Download....please wait.", Toast.LENGTH_LONG).show();
async.new Connect().execute("166.62.10.29", "M1_May_15.pdf");
} else {
Toast.makeText(Maths1.this, "No internet connection \n Please try again", Toast.LENGTH_SHORT).show();
}
}
}
});
d15.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
File pdf = new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/GeniusPhysicsandMaths/" + "M1_Dec_15.pdf");
if (pdf.exists()) {
ViewPDF();
} else {
if (isNetworkConnected() == true) {
async.new Connect().execute("166.62.10.29", "M1_Dec_15.pdf");
Toast.makeText(Maths1.this, "Starting Download....please wait.", Toast.LENGTH_LONG).show();
ViewPDF();
} else {
Toast.makeText(Maths1.this, "No internet connection \n Please try again", Toast.LENGTH_SHORT).show();
}
}
}
});
}
private boolean isNetworkConnected() {
ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
return cm.getActiveNetworkInfo() != null;
}
public void ViewPDF()
{
Intent i=new Intent(Maths1.this,PdfViewer.class);
SavePreferences("name", "M1_Dec_15.pdf");
startActivity(i);
}
private void SavePreferences(String key, String value) {
SharedPreferences sharedPreferences = getPreferences(MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString(key, value);
editor.commit();
}
}
pdf viewer code:
package com.materialdesign.geniusmathsandphysics;
import android.content.SharedPreferences;
import android.os.Environment;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import java.io.File;
public class PdfViewer extends AppCompatActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.pdfviewer);
com.joanzapata.pdfview.PDFView Pdf = (com.joanzapata.pdfview.PDFView) findViewById(R.id.pdfview1);
//String filepath=getIntent().getExtras().getString("name");
String file=showPreferences("name");
File fd = new File(file);
//Log.i("hhhhdsdada",filepath);
Pdf.fromFile(fd).defaultPage(1).swipeVertical(true).load();
}
private String showPreferences(String key){
SharedPreferences sharedPreferences = getPreferences(MODE_PRIVATE);
String savedPref = sharedPreferences.getString(key, "");
File files= new File(Environment.getExternalStorageDirectory().getAbsolutePath() + "/GeniusPhysicsandMaths/"+savedPref);
String file=files.toString();
return file;
}
}
this is my logcat:
FATAL EXCEPTION: AsyncTask #3 Process: com.materialdesign.geniusmathsandphysics, PID: 18871
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:300)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
at java.util.concurrent.FutureTask.run(FutureTask.java:242)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
at java.lang.Thread.run(Thread.java:818)
Caused by: java.lang.RuntimeException: PDF file is corrupted
at org.vudroid.pdfdroid.codec.PdfDocument.open(Native Method)
at org.vudroid.pdfdroid.codec.PdfDocument.openDocument(PdfDocument.java:28)
at org.vudroid.pdfdroid.codec.PdfContext.openDocument(PdfContext.java:18)
at org.vudroid.core.DecodeServiceBase.open(DecodeServiceBase.java:59)
at com.joanzapata.pdfview.DecodingAsyncTask.doInBackground(DecodingAsyncTask.java:52)
at com.joanzapata.pdfview.DecodingAsyncTask.doInBackground(DecodingAsyncTask.java:31)
at android.os.AsyncTask$2.call(AsyncTask.java:288)
at java.util.concurrent.FutureTask.run(FutureTask.java:237)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112) 
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587) 
at java.lang.Thread.run(Thread.java:818) 
i am using joanzapata library to view pdf files.for that i am trenter code hereying to pass that file name from maths 1 class on button click listner. The library takes file as input so i had to convert string to file previously using intent i was getting null pointer exception but now with shared preferences i am getting this error. Please help me to solve it. Thanks in advance.