This sample shows how to protect your PDF document with one or more certificates using the AES 256-bit encryption algorithm in C# and VB.NET.
Create an instance of the PdfPublicKeyEncryptionHandler class using an X509Certificate2 certificate or a key store. If you would like to limit permissions for the document, then use a constructor overload that accepts an instance of the PdfPermissions class.
Then specify PdfEncryptionAlgorithm.Aes256Bit for the Algorithm property of the handler. Finally, assign the handler to the PdfSaveOptions.EncryptionHandler property to protect the output PDF file with advanced protection offered by AES 256-bit encryption.
To add more recipients, please use AddRecipient and/or AddOwner methods of the PdfPublicKeyEncryptionHandler class.