Like a hand-written signature, a digital signature proves that you have created or agree with a document. By electronically signing a PDF, you achieve two goals:

The recipient of the document can be sure that it came from you
Once the certificate is validated, the PDF viewer will use the public key that is found in the certificate to verify the signature. A signature applied using your private key can only be decrypted by using its corresponding public key. Thus, the recipient can be sure that the document came from you and that it has not been changed since it was signed.

To sign a PDF document, you need a digital ID. See obtaining a digital ID.

What is a digital ID and how does it work?

To sign a document, you need a digital ID, sometimes called a certificate. A digital ID is a file on your computer. Security software can use this file to prove your identity to other people. You can use a digital ID not only for signing documents but also for Client Authentication (when you log in to your bank account) and for Encryption.

Key Pairs

Digital IDs contain a pair of keys: a public key and a private key. Keys are just long sequences of digits much like a very strong password. Keys are like your ordinary keys except that one key is used to lock the content and the other is used to unlock it.

When you create a self-signed digital ID or buy one from a third party certificate authority, you receive a pair of keys - a public key and a private key.

The digital ID containing the key pair is either automatically installed on your system upon obtaining (you can export the digital ID later into a .pfx file), or the digital ID comes recorded on a smart card or a hardware token.

Private Key

When you sign a document, your software application uses this private key. It is usually password protected and can be stored on your computer in PKCS #12 (.pfx ) file format, on a smart card, in the Windows certificate store, or on a server. All of these options are available when signing PDFs with Smart PDF Converter.

Public Key and Certificate

Your certificate contains your public key and your name. You send the certificate to others and they use it to validate your signature or identity. Typically, you don't need to send the certificate explicitly but it is automatically attached to your signature. So when the recipient opens a PDF signed by you with a PDF viewer such as Adobe Reader, the PDF viewer software will:

Verify that the attached certificate is valid. Certificates purchased from a Certificate Authority are signed by that authority so they can validate your identity. They verify that the public key belongs to you.

Self-signed certificates are not signed, so they do not offer to the recipient of the document any proof of the signer's identity.
Once the certificate is validated, the PDF viewer will use the public key that is found in the certificate to verify the signature. A signature applied using your private key can only be decrypted by using its corresponding public key. Thus, the recipient can be sure that the document came from you and that it has not been changed since it was signed.

Obtaining a digital ID

You can buy a digital ID from a third party authorization authority, get one from your own organization, or create a self-signed one.

Creating a self-signed digital ID

Self-signed IDs are good for personal use or for documents sent within your organization.

To create a self-signed ID with Smart PDF Converter, click Sign on the Tools tab. Open a PDF file to sign, and click Sign. A new window appears. Click Create Self-Signed ID and follow the steps to enter your name and other info. The software installs the new certificate in the Windows Certificate Store, and you will be able to use it to sign your PDF files, email messages, and other documents.

Buying a digital ID

Most sensitive transactions between businesses require a digital ID issued by a third-party certificate authority (CA). Because you need to prove your identity to others, you need a certificate authority that others will trust.

Here is a list of some of the popular certificate authorities.

Trusted Signatures and Validating Signatures

Self-signed certificates are not purchased from a certificate authority and must be sent to the recipient securely. The recipient has to trust a self-signed certificate explicitly. If the signer’s digital ID has not been explicitly trusted, the signature validity will be unknown. The recipient will see the following icon . The recipient can then add the certificate to their list of trusted identities.

To add a particular certificate to your list of trusted identities, open the document in Adobe Reader, right-click on a signature, select Show Signature Properties and then Show Certificate. Then go to the Trust tab and click Add to Trusted Identities.