To Protect a File with a Certificate

From the Protect dialog, click the With Certificate button.
Do one of the following:

To select a certificate from your personal certificate store, click the Choose Certificate icon and choose a certificate for the Certificate dialog.

or
To select a certificate file, click the Choose Certificate file icon and choose a certificate file from the Certificate files dialog.
Click OK.
The file is protected with the certificate.

CAUTION: If you try to protect a file with a certificate sent to you by someone else it will prevent you from accessing your own file in future, and you will receive the following warning message:

Note: It is a good idea to protect your private key against loss. Check with Microsoft for the best method to protect your private key.