Thanks for your update.
But it is not my need. I still do not know when the user back from the PDF view to my app view (by pressed back button).
At last I implemented it myself by modify the cordova plugin source code.
To put it simply,
1. Modify file RadaeePDFPlugin.java;
comment:
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
change:
mContext.startActivity(intent);
to:
this.cordova.startActivityForResult((CordovaPlugin)this, intent, 0);
and add:
@Override
    public void onActivityResult(final int requestCode, final int resultCode, final Intent data) {
        if(requestCode == 0) {
            if(resultCode == cordova.getActivity().RESULT_OK) {
                Bundle extras = data.getExtras();// Get data sent by the Intent
                String information = extras.getString("fileState"); // data parameter will be send from the other activity.
                PluginResult resultado = new PluginResult(PluginResult.Status.OK, information);
                resultado.setKeepCallback(true);
                PUBLIC_CALLBACKS.sendPluginResult(resultado);
                return;
            }
            else if(resultCode == cordova.getActivity().RESULT_CANCELED) {
                PluginResult resultado = new PluginResult(PluginResult.Status.OK, "canceled action, process this in javascript");
                resultado.setKeepCallback(true);
                PUBLIC_CALLBACKS.sendPluginResult(resultado);
                return;
            }
        }
        // Handle other results if exists.
        super.onActivityResult(requestCode, resultCode, data);
    }
2. Modify file PDFViewAct.java;
change function 'onBackPressed' to:
@Override
public void onBackPressed()
{
    if(m_controller == null || m_controller.OnBackPressed())
    {
        if(m_modified) {
            TextView txtView = new TextView(this);
            txtView.setText("Document modified\r\nDo you want save it?");
            new AlertDialog.Builder(this).setTitle("Exiting").setView(
                    txtView).setPositiveButton("Yes", new DialogInterface.OnClickListener()
                    {
                       @Override
                       public void onClick(DialogInterface dialog, int which)
                       {
                            m_doc.Save();
                           
                            Intent mIntent = new Intent();
                            mIntent.putExtra("fileState", "modifiedAndSaved");
                            PDFViewAct.this.setResult(RESULT_OK, mIntent);
                            PDFViewAct.super.onBackPressed();
                       }
                   }).setNegativeButton("No", new DialogInterface.OnClickListener()
                   {
                       @Override
                       public void onClick(DialogInterface dialog, int which)
                       {
                            Intent mIntent = new Intent();
                            mIntent.putExtra("fileState", "modifiedAndNotSaved");
                            PDFViewAct.this.setResult(RESULT_OK, mIntent);
                            PDFViewAct.super.onBackPressed();
                       }
                   }).show(); 
        }
        else {
            Intent mIntent = new Intent();
            mIntent.putExtra("fileState", "notModified");
            setResult(RESULT_OK, mIntent);
            super.onBackPressed();
        }
    }
}
Now when user back from PDF view, I can get file state and do my process.