Allow to password-protect a document, so that it cannot be shown (read) without the password. Also, enable opening encrypted documents.

Workaround for encrypting: PdfProcessing can be used to encrypt the document:

using (MemoryStream ms = new MemoryStream())
{
    PdfFormatProvider pdfProcessingFormatProvider = new PdfFormatProvider();
    Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider pdfFormatProvider = new Telerik.Windows.Documents.Flow.FormatProviders.Pdf.PdfFormatProvider();
    pdfFormatProvider.Export(flowDocument, ms);
    RadFixedDocument fixedDocument = pdfProcessingFormatProvider.Import(ms);

    using (Stream output = new FileStream(outputPath, FileMode.OpenOrCreate))
    {
        pdfProcessingFormatProvider.ExportSettings.IsEncrypted = true;
        pdfProcessingFormatProvider.ExportSettings.UserPassword = "pass";
        pdfProcessingFormatProvider.ExportSettings.ImageQuality = ImageQuality.Medium;
        pdfProcessingFormatProvider.Export(fixedDocument, output);
    }
}