The software should be the one deciding what format, not the TWAIN driver. It would not make sense for the driver to decide since it would not know what format the requesting app can read. It wouldn't be helpful to have TWAIN set to pdf when the application supports .pdf or .doc for instance.
Many programs will decide a single page doc that doesn't have OCR should be .jpg or .tiff.
You completely left out what 3rd party software as well as what printer.