Simple flow with a Digital Signature

Hi,
I have a a dynamic PDF document which includes a document signature on it. The flow begins with starting the process which has task manager end point. The starter of the process fills the form and sign the document and commits. Then another user login to workspace and open the To Do list and see the filled form. However digital signature is not preserved, missing. I have defined my pdf form as xfaForm.
How can I create a flow preserving the digital signature?

Thanks,
Asiye