If you send a document or email signed with a PKI certificate, can the recipient open it up and see the signature without having a PKI certificate? If the recipient needs a PKI certificate is there anything else the recipient needs?

Answer: 

The recipient of a signed document does not need to have a PKI certificate of their own, but they will need software to verify the digital signature and view the certificate. If the recipient's email system is capable of accepting PKI digitally signed emails or documents, the recipient will be able to open the document and verify the signature and view the certificate.

If the sender wants to encrypt an email to send to the recipient, the recipient will need a PKI encryption [public] certificate and will have to provide it to the sender. Then, when the message is received, the recipient's email system can also decrypt the message.

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