Security of signing PDF forms.

Hi Łukasz374361620df9,

Thank you for reaching out, and sorry about the delayed response.

If you send the document for signature, you can assign form fields to individuals. Only the assigned individuals can fill out those form fields.

After the document is signed by all recipients and submitted, there are no changes can be made to that document. 

Hope that answers your doubts.

Let us know if you are referring to something else or have any questions.

Thanks,

Meenakshi

What exact behavior do you expect when you say "blocked"?
What you can expect is that a PDF signature validator that supports the MDP (Modification Detection and Prevention) mechanism will show the signature as invalid after those locked fields have been changed somehow.  Normal form filling tools also should not allow their user to change those fields.

What you cannot expect, though, is that it is impossible to change those fields. The format PDF by design allows adding arbitrary changes in incremental updates.