How to convert PPT to PDF in ASP.NET without using 3rd party dlls?

using Microsoft.Office.Interop.PowerPoint;

protected void Page_Load(object sender, EventArgs e)
        {
            string szInputFile = @"D:\XYZ\PPTFile.pptx";
            string szOutputFile = @"D:\XYZ\DemoPDF.pdf";
            int Errors = 0;
            Microsoft.Office.Interop.PowerPoint._Application objPPApp = null;
            Presentation objPPDoc = null;
            try
            {
                objPPApp = new Application();
                objPPDoc = objPPApp.Presentations.Open(szInputFile);
                objPPDoc.SaveAs(szOutputFile, PpSaveAsFileType.ppSaveAsPDF);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            finally
            {
                if ((objPPDoc != null))
                {
                    objPPDoc.Close();
                    objPPDoc = null;
                }
                if ((objPPApp != null))
                {
                    objPPApp.Quit();
                    objPPApp = null;
                }
            }
        }