If you don't want the image of your original PDF to be modified in any way after the recognition, you should use the following scenario:

1. Open your PDF in FineReader 15 PDF-Editor;
2. Press Recognize on the toolbar or press Ctrl+Shift+R on your keyboard;
3. Choose OCR language, uncheck Correct page orientation and Deskew images and correct image resolution options if they are checked, press Recognize button;
   
4. Press File > Save or Ctrl+S.

If you use a different scenario of recognition the original image will be modified according to export settings.