You will not be able to use Digital File Pro to validate the file type if it is not an image. Instead you should make use of a Server Validation for the file type to ensure that it is a pdf. You would then have a standard Digital File Pro upload that is not checking for the file to be an image.
This will allow you to confirm that the file is a pdf before it is uploaded, then the upload will occur. You can then use any of the bindings from the upload in a db insert or update by going to the bindings and expanding the upload entry.
To apply the server validation you will need to have Validation Toolkit or CSS Form Builder installed.