System.Windows.Forms.RichTextBox rtb;
        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            var directory = System.IO.Path.GetDirectoryName(System.Reflection.Assembly.GetEntryAssembly().Location);
            var file = System.IO.Path.Combine(directory, "contractText.rtf");

            rtb = new System.Windows.Forms.RichTextBox();
            rtb.LoadFile(file);
            rtb.Text.Replace("idContract", IdContractglob.ToString());
            rtb.Text.Replace("date", dateFrom);
            rtb.Text.Replace("FIOSOB", tbFIOSOB.Text);
            rtb.Text.Replace("SerNomPassportSOB", tbSerNomPassportSOB.Text);
            rtb.Text.Replace("KemVadanDataSOB", tbKemVadanDataSOB.Text);
 
            var s = rtb.Text;// вот тут смотрю, текст без замены отображает

            PrintDialog printDialog = new PrintDialog();
            PrintDocument documentToPrint = new PrintDocument();
            if (printDialog.ShowDialog() == true)
            {
                StringReader reader = new StringReader(rtb.Text);
                documentToPrint.PrintPage += new PrintPageEventHandler(DocumentToPrint_PrintPage);
                documentToPrint.Print();
            }
        }

        private void DocumentToPrint_PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
        {
            rtb = new System.Windows.Forms.RichTextBox();
            StringReader reader = new StringReader(rtb.Text);
            float LinesPerPage = 0;
            float YPosition = 0;
            int Count = 0;
            float LeftMargin = e.MarginBounds.Left;
            float TopMargin = e.MarginBounds.Top;
            string Line = null;
            Font PrintFont = this.rtb.Font;
            SolidBrush PrintBrush = new SolidBrush(System.Drawing.Color.Black);

            LinesPerPage = e.MarginBounds.Height / PrintFont.GetHeight(e.Graphics);

            while (Count < LinesPerPage && ((Line = reader.ReadLine()) != null))
            {
                YPosition = TopMargin + (Count * PrintFont.GetHeight(e.Graphics));
                e.Graphics.DrawString(Line, PrintFont, PrintBrush, LeftMargin, YPosition, new StringFormat());
                Count++;
            }

            if (Line != null)
            {
                e.HasMorePages = true;
            }
            else
            {
                e.HasMorePages = false;
            }
            PrintBrush.Dispose();
        }