PendingIntent does not send Intent extras

Using PendingIntent.FLAG_CANCEL_CURRENT not a good solution because of inefficient use of memory. Instead use PendingIntent.FLAG_UPDATE_CURRENT.

Use also Intent.FLAG_ACTIVITY_SINGLE_TOP (the activity will not be launched if it is already running at the top of the history stack).

Intent resultIntent = new Intent(this, FragmentPagerSupportActivity.class).
                    addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);  
resultIntent.putExtra(FragmentPagerSupportActivity.PAGE_NUMBER_KEY, pageNumber);

PendingIntent resultPendingIntent =
                PendingIntent.getActivity(
                        this,
                        0,
                        resultIntent,
                        PendingIntent.FLAG_UPDATE_CURRENT
                );

Then:

@Override
    protected void onCreate(Bundle savedInstanceState) {
        try {
            super.onCreate(savedInstanceState);

            int startPageNumber;
            if ( savedInstanceState != null)
            {
                startPageNumber = savedInstanceState.getInt(PAGE_NUMBER_KEY);
//so on

It should work now.

If you still have not expected behaviour, try to implement void onNewIntent(Intent intent) event handler, that way you can access the new intent that was called for the activity (which is not the same as just calling getIntent(), this will always return the first Intent that launched your activity.

@Override
protected void onNewIntent(Intent intent) {
    int startPageNumber;

    if (intent != null) {
        startPageNumber = intent.getExtras().getInt(PAGE_NUMBER_KEY);
    } else {
        startPageNumber = 0;
    }
}