Q: We know that standard interactive form fields can have multiple widgets associated with them. In this case, all “linked” widgets in the document will contain the same form field data & have the same appearance stream. But how about digital signature fields? Do they allow multiple widgets to be associated with them?
Answer: The answer is no.
It’s important to note that a digital signature applies to the whole PDF document even if it only renders on one page (typically the last or first page of the document). And if any content is modified on any page of the document (not only on the page that renders the signature), the digital signature will be invalidated.
According to the PDF 2.0 specifications:
“The location of a signature within a document can have a bearing on its legal meaning. For this reason, signature fields shall never refer to more than one annotation.”
This Adobe forum seems to indicate that Adobe used to support this in older version but no longer does:
Qoppa’s PDF rendering engine will handle a document created with a signature field linked to multiple widgets and will render the signature appearance stream the same across the widgets, in the same way other PDF viewing applications do.
Qoppa’s public PDF API. however, does not allow to link different widgets to the same signature field. The same goes for all our derived PDF applications. We are not planning to add the ability to do so, considering that this would go against the PDF specifications.
Workaround: You can apply the signature images to each page (using flattened rubber stamps or button image fields) and then sign the PDF document with a digital signature (assuming this is the first signature on the document).