private Table _MainTable;
    private Unit PageWidth = new Unit(11, UnitType.Inch);
    private Unit PageHeight = new Unit(8.5, UnitType.Inch);
    private Unit ItemHeight = new Unit(.25, UnitType.Inch);
    private Unit TenPointFont = new Unit(10, UnitType.Point);
    private Unit TableWidth;

    public class TestReportObject
    {
        public int? RowID { get; set; }
        public string Description { get; set; }
        public string LongDescription { get; set; }
    }

        private void AddTestColumn(int colIndex, string header, string fieldName, Unit colWidth)
        {
            TableGroup tableGroupColumn = new TableGroup();
            _MainTable.ColumnGroups.Add(tableGroupColumn);
            _MainTable.Body.Columns.Add(new Telerik.Reporting.TableBodyColumn(colWidth));

            Telerik.Reporting.TextBox columnHeaderTextBox;
            Telerik.Reporting.TextBox cellTextBox;

            columnHeaderTextBox = new Telerik.Reporting.TextBox();
            columnHeaderTextBox.Style.BackgroundColor = Color.LemonChiffon;
            columnHeaderTextBox.Style.BorderStyle.Default = BorderType.Solid;

            string name = header;
            columnHeaderTextBox.Value = name;
            columnHeaderTextBox.Name = name;

            columnHeaderTextBox.Size = new SizeU(colWidth, ItemHeight);
            columnHeaderTextBox.Style.Font.Size = TenPointFont;
            columnHeaderTextBox.Style.TextAlign = HorizontalAlign.Center;
            tableGroupColumn.ReportItem = columnHeaderTextBox;
            columnHeaderTextBox.TextWrap = true;
            columnHeaderTextBox.Height = ItemHeight;

            _MainTable.Items.Add((ReportItemBase)columnHeaderTextBox);

            cellTextBox = new Telerik.Reporting.TextBox();
            cellTextBox.Style.BorderColor.Default = Color.Black;
            cellTextBox.Style.BorderStyle.Default = BorderType.Solid;

            cellTextBox.Value = $"=Fields.{fieldName}";
            cellTextBox.Name = name;

            cellTextBox.Size = new SizeU(colWidth, ItemHeight);
            cellTextBox.Style.TextAlign = HorizontalAlign.Left;
            cellTextBox.Style.Font.Size = TenPointFont;
            cellTextBox.Style.VerticalAlign = VerticalAlign.Middle;
            cellTextBox.Height = ItemHeight;
            cellTextBox.TextWrap = true;
            _MainTable.Body.SetCellContent(0, colIndex, cellTextBox);

        }

        private Telerik.Reporting.Report BuildTestReport()
        {
            Telerik.Reporting.Report TestReport = new Report();

            #region page settings (making the assumption of landscaped 8.5 x 11 paper)

            TestReport.PageSettings.Landscape = true;
            TestReport.PageSettings.PaperSize = new SizeU(PageWidth, PageHeight);

            double BottomMarginSize = ExportParameters.BottomMargin == null ? .5 : ExportParameters.BottomMargin.Value;
            TestReport.PageSettings.Margins.Bottom = new Unit(BottomMarginSize, UnitType.Inch);

            double LeftMarginSize = ExportParameters.LeftMargin == null ? 1 : ExportParameters.LeftMargin.Value;
            TestReport.PageSettings.Margins.Left = new Unit(LeftMarginSize, UnitType.Inch);

            double RightMarginSize = ExportParameters.RightMargin == null ? 1 : ExportParameters.RightMargin.Value;
            TestReport.PageSettings.Margins.Right = new Unit(RightMarginSize, UnitType.Inch);

            double TopMarginSize = ExportParameters.TopMargin == null ? .5 : ExportParameters.TopMargin.Value;
            TestReport.PageSettings.Margins.Top = new Unit(TopMarginSize, UnitType.Inch);

            TableWidth = PageWidth - (new Unit(LeftMarginSize + RightMarginSize, UnitType.Inch));

            #endregion

            Unit remainingPageWidth = TableWidth;
            Unit fullTableWidth = new Unit(11, UnitType.Inch);

            _MainTable = new Table();
            _MainTable.Location = new PointU(new Unit(0, UnitType.Inch), new Unit(0, UnitType.Inch));
            _MainTable.Size = new SizeU(fullTableWidth, ItemHeight);

            // Body
            DetailSection ExportDetail = new DetailSection();


            int RowCountTotal = 50000;
            var rowList = new List<TestReportObject>();
            for (int rows = 0; rows < RowCountTotal; rows++)
            {
                var reportObj = new TestReportObject();
                reportObj.RowID = rows;
                reportObj.Description = $"Row {rows}";
                reportObj.LongDescription = $"This is the long description for row {rows}";
                rowList.Add(reportObj);
            }

            _MainTable.DataSource = rowList;
            _MainTable.Style.VerticalAlign = VerticalAlign.Middle;
            _MainTable.ColumnHeadersPrintOnEveryPage = true;
            _MainTable.Name = "mainTable";

            // default grouping has to be added to allow data to appear
            Telerik.Reporting.TableGroup tableGroup = new Telerik.Reporting.TableGroup();
            tableGroup.Groupings.AddRange(new Telerik.Reporting.Grouping[] { new Telerik.Reporting.Grouping("") });
            tableGroup.Name = "DetailGroup";
            _MainTable.RowGroups.Add(tableGroup);

            #region Define columns and cells within the main table

            Unit theColumnWidth = _MainTable.Width / 3;  // number of columns
            AddTestColumn(0, "Row ID", "RowID", theColumnWidth);
            AddTestColumn(1, "Description", "Description", theColumnWidth);
            AddTestColumn(2, "Long Description", "LongDescription", theColumnWidth);


            #endregion

            ExportDetail.Items.AddRange(new ReportItemBase[] { _MainTable });
            TestReport.Items.Add((ReportItemBase)ExportDetail);

            return TestReport;
        }


        /// <summary>
        /// Generates the bytes of the export file
        /// </summary>
        private byte[] ExportBytes()
        {
            Report rpt = BuildTestReport();

            InstanceReportSource rptSource = new InstanceReportSource();
            rptSource.ReportDocument = rpt;

            using (new ContextScope(new Hashtable()))
            {
                Hashtable deviceInfo = new Hashtable();
                deviceInfo["FontEmbedding"] = "Subset";

                Telerik.Reporting.Processing.ReportProcessor RP = new Telerik.Reporting.Processing.ReportProcessor();

               // ExportFileTypeString is UI selection, pdf, csv, etc.
                byte[] buffer = RP.RenderReport(ExportFileTypeString, rptSource, deviceInfo).DocumentBytes;

                return buffer;
            }
        }

deviceInfo.Add("OwnerPassword", "myPassword");
deviceInfo.Add("UserPassword", "myPassword");

deviceInfo.Add("SignCertFilename", "C:\\Temp\\my.cer");
deviceInfo.Add("SignCertPassword", "myPassword");