Private and public keys are used in two main encryption systems: Symmetric and Asymmetric.
This system uses only private keys. This requires the private key (code) to be installed on specific computers that will be used for exchanging messages between certain users. The system works pretty much like two best friends using a decoder ring to send secret messages to each other. Both friends know which code they are using and thus, only they will have the key to crack and encode secret messages.
The Asymmetric Encryption system uses both the private and public keys. The private key is for yourself and the public key is published on line for others to see.
They use the public key to access the encryption code that corresponds to your private key. So, if you are sending an encrypted message to Susan which you do not want others to see, you would use her public key to encrypt it. She will be able to decrypt it with her own corresponding private key. Likewise, if she sends a message to you, she uses your public key to encrypt the message and you would use your private key to decrypt it.
Digital IDs are ideal because they can't be easily forged. They allow for a more electronically secured communication line because they enable you to make use of these encryption systems. By applying them to your PDF files after the PDF creation process, you are ensuring their security.