public class FacturaActivity extends Activity {

    (...)

    public void downloadInvoice(View view) {
        File file = new File(Environment.getExternalStorageDirectory().getAbsolutePath() +"/"+ filename);
        Intent intent = new Intent(Intent.ACTION_VIEW);
        intent.setDataAndType(Uri.fromFile(file),"application/pdf");
        intent.setFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);
        startActivity(intent);
    }
}

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

<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="8" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme"
    android:name="###.MyApplication" > <!--cant show complete name-->
    <activity
        android:name="###.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity
        android:name=".FacturaActivity" >
    </activity>

</application>

07-03 15:49:13.094: E/AndroidRuntime(1032): FATAL EXCEPTION: main
07-03 15:49:13.094: E/AndroidRuntime(1032): java.lang.IllegalStateException: Could not execute method of the activity
(...)
07-03 15:49:13.094: E/AndroidRuntime(1032): Caused by: android.content.ActivityNotFoundException: No Activity found to handle Intent { act=android.intent.action.VIEW dat=file:///mnt/sdcard/201209_F2012212782.PDF typ=application/pdf flg=0x40000000 }
07-03 15:49:13.094: E/AndroidRuntime(1032):     at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:1408)
07-03 15:49:13.094: E/AndroidRuntime(1032):     at android.app.Instrumentation.execStartActivity(Instrumentation.java:1378)
07-03 15:49:13.094: E/AndroidRuntime(1032):     at android.app.Activity.startActivityForResult(Activity.java:2817)
07-03 15:49:13.094: E/AndroidRuntime(1032):     at android.app.Activity.startActivity(Activity.java:2923)

File file = new File(Environment.getExternalStorageDirectory().getAbsolutePath() +"/"+ filename);
Intent target = new Intent(Intent.ACTION_VIEW);
target.setDataAndType(Uri.fromFile(file),"application/pdf");
target.setFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);

Intent intent = Intent.createChooser(target, "Open File");
try {
    startActivity(intent);
} catch (ActivityNotFoundException e) {
    // Instruct the user to install a PDF reader here, or something
}

String dir="/Attendancesystem";

 public void displaypdf() {

        File file = null;
            file = new File(Environment.getExternalStorageDirectory()+dir+ "/sample.pdf");
        Toast.makeText(getApplicationContext(), file.toString() , Toast.LENGTH_LONG).show();
        if(file.exists()) {
            Intent target = new Intent(Intent.ACTION_VIEW);
            target.setDataAndType(Uri.fromFile(file), "application/pdf");
            target.setFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);

            Intent intent = Intent.createChooser(target, "Open File");
            try {
                startActivity(intent);
            } catch (ActivityNotFoundException e) {
                // Instruct the user to install a PDF reader here, or something
            }
        }
        else
            Toast.makeText(getApplicationContext(), "File path is incorrect." , Toast.LENGTH_LONG).show();
    }