About digital signatures

A digital signature, like a conventional handwritten signature, identifies the person signing a document. Unlike a handwritten signature, a digital signature is difficult to forge because it contains encrypted information that is unique to the signer and easily verified.

Most digital signatures are referred to as approval signatures. Signatures that certify an Adobe® PDF are called certifying signatures. Only the first person to sign a PDF (most often, the author) can add a certifying signature. A certifying signature attests to the contents of the document. It also allows the signer to specify the types of changes allowed for the document to remain certified. Changes to the document are shown in the Signatures panel.

To sign a document, you must obtain a digital ID or create a self-signed digital ID in Adobe Acrobat®. The digital ID contains a private key and a certificate (or public key). The private key is used to add the digital signature, and the certificate that you share with people who use it to validate your signature.

When you apply a digital signature, Acrobat uses a hashing algorithm to generate a message digest, which it encrypts using your private key. Acrobat embeds the encrypted message digest in the PDF, along with details from your certificate, an image of your signature, and a version of the document at the time it was signed.

Note: For the latest information about digital signatures, choose Help > Online Support > Knowledgebase to open the Adobe Acrobat support page on the Adobe website. Then search for “digital signatures.”

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Digital signature in a PDF form