PDF and PDF / A documents allow two types of document signatures: the invisible and the visible signature.

The invisible signature does not change the appearance of the electronic document. Therefore, this form of signature is suitable for all processes where the existence of the security features of electronic signatures are needed, but a change in appearance will not be allowed in any case.

The visible signature enables the integration of visualization data into the PDF document, and shows the presence of an electronic signature to the user through a separate visual representation in the document. For both types of signature exists a representation object, the Appearance Stream. In the signature object of the invisible signature the Appearance Stream is empty.