An electronic signature differentiates from a handwritten signature in that it is not visible, but appears as a number or a set of numbers. In fact, the signature operation is applied to a file and produces binary information often called an electronic signature or a cryptographic. This is not only linked to the file, but also to the person who signed it.
That being said, some software and particularly Adobe Reader automatically verify each signature when the document is opened and attach a visual validation message that allows an electronic signature to materialise.