string pdfFile = "...";
FastReport.PdfSigning pdfSigning = new FastReport.PdfSigning();
pdfSigning.SignatureBehavior = MercatorUi.SignatureBehaviorEnum.Optional;
pdfSigning.CertificateFile = @"<MainDir\MyPFX.pfx";
pdfSigning.CertificateFilePassword = "1234";
pdfSigning.HideInterfaceIfFile = true;
MercatorUi.Reporting.ReportingStatic.Reporting.SignPdf(pdfFile, pdfSigning);