Hi Brent,
Thank you for using Syncfusion products,
We can add the security settings to the document as follow.
PdfDocument doc=new PdfDocument();
// Set encryption key.
doc.Security.KeySize = PdfEncryptionKeySize.Key128Bit;
doc.Security.Algorithm = PdfEncryptionAlgorithm.AES;
// Setting username and password to protect the document.
doc.Security.OwnerPassword = "syncfusion";
doc.Security.UserPassword = "password";
// Allow print with full quality.
doc.Security.Permissions = PdfPermissionsFlags.Print | PdfPermissionsFlags.FullQualityPrint;
doc.Save("sample.pdf");
Please visit the below link for more information about the Pdf security.
Please let us know if you have any concern,
With Regards,
Praveen