The Document has been altered or corrupted since the Signature was applied

Hi,

I have a smartcard with a digital signature that designed to sign on documents,
If I tried to add a digital signature to a Word document using MS Word I'm able to do so successfully,
but if I try to add a digital signature to a PDF file using Adobe Reader, after I save the new signed file I immediately see a message that the signature is INVALID and the reason is that "The Document has been alterted or corrupted since the Signature was applied",

but it is the immediate file that has been saved when I add the digital signature,
I can reproduce it and I'm unable to add a digital signature to any PDF file without getting this error,

What could be the reason for this behaviour?

Thanks in advance!