A friend recently asked me about this. He had signed an PDF, but was still able to make changes to the form fields. It makes sense that “Nothing happens when signed.” On the other hand  there is a presumption, as my friend had, that digitally signing locks the document.

Click here to download a PDF tutorial and sample.

Here is the default "Nothing happens when signed." Clicking Mark as read-only" locks all the form fields.
 


The example below shows a digitally signed form that is still fillable. The bottom form shows a second signature that locks all the fields.