Hi,
I have no idea why you lose your digital signature. I have tested on my side, it is OK.
So I am afraid I couldn't give some useful advice, but I think if this issue still happens, you can try to create a digital signature in PDF. How To Create a Digital Signature in PDF, you can refer to this article:
http://www.ehow.com/how_4809936_digital-signature-pdf.html
Hope that helps.