In short, two possibilities:
a) In the action where your first recipient sends the document off, set the fields to be protected to readonly.
b) have first recipient apply a digital signature, and set the according signature field so that the fields you want to have protected are set to readonly. This step would also allow to reveal manipulations.
There are other possibilities, which would be server-supported, and therefore require according infrastructure.
Hope this can help.
Max Wyss.