Hi  mashispano,
Refer below sample. Add the reference of Windows.Forms from add reference window in your project for reading the RTF file.
Namespaces
C#
using PdfSharp.Drawing;
using PdfSharp.Pdf;
using System.IO;
VB.Net
Imports PdfSharp.Drawing
Imports PdfSharp.Pdf
Imports System.IO
Code
C#
protected void Page_Load(object sender, EventArgs e)
{
    // Create a new PDF document.
    using (PdfDocument document = new PdfDocument())
    {
        // Adding an empty page.
        PdfPage page = document.AddPage();
        // Get an XGraphics object for drawing.
        using (XGraphics gfx = XGraphics.FromPdfPage(page))
        {
            // Creating font.
            XFont font = new XFont("Time New Roman", 15, XFontStyle.Regular);
            // Reading the RTF file using RichTextBox.
            System.Windows.Forms.RichTextBox richTextBox = new System.Windows.Forms.RichTextBox();
            richTextBox.Rtf = File.ReadAllText(Server.MapPath("~/templates/sample.rtf"));
            string text = richTextBox.Text;
            // Drawing the text.
            gfx.DrawString(text, font, XBrushes.Black, new XRect(0, 0, page.Width, page.Height), XStringFormats.Center);
            // Saving the document.
            string filename = Server.MapPath("~/export/" + "SAMPLE_" + DateTime.Now.ToString("yyyyMMddHHmmss") + ".pdf");
            document.Save(filename);
        }
    }
}
VB.Net
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    ' Create a new PDF document.
    Using document As PdfDocument = New PdfDocument()
        ' Adding an empty page.
        Dim page As PdfPage = document.AddPage()
        ' Get an XGraphics object for drawing.
        Using gfx As XGraphics = XGraphics.FromPdfPage(page)
            ' Creating font.
            Dim font As XFont = New XFont("Time New Roman", 15, XFontStyle.Regular)
            ' Reading the RTF file using RichTextBox.
            Dim richTextBox As Windows.Forms.RichTextBox = New Windows.Forms.RichTextBox()
            richTextBox.Rtf = File.ReadAllText(Server.MapPath("~/templates/sample.rtf"))
            Dim text As String = richTextBox.Text
            ' Drawing the text.
            gfx.DrawString(text, font, XBrushes.Black, New XRect(0, 0, page.Width, page.Height), XStringFormats.Center)
            ' Saving the document.
            Dim filename As String = Server.MapPath("~/export/" & "SAMPLE_" & DateTime.Now.ToString("yyyyMMddHHmmss") & ".pdf")
            document.Save(filename)
        End Using
    End Using
End Sub