afaik reading pdf content is something you can't do with standard Nintex actions. I know some providers offer pdf actions for nintex and there maybe a way to write a custom action to do this, but nothing out of the box available.
Apart from that, the workflow would be rather simple. You start it on Item-Modification and as a first workflow action you check if your pdf signature field is empty. If it's not, you just write the value back to a column in your list, otherwise you do nothing.