This is the C# routine that I use:
       private string RenderAsPDF(Microsoft.Reporting.WinForms.ServerReport
Report)
       {
           // this.Text is the name of the report as shown on the form's
caption
           string TempFile = Path.GetTempPath() + this.Text + ".pdf";
           string MimeType;
           string FileExt;

           DeleteFile(TempFile);

           // create an output FileStream for the pdf file. Render the
report to this stream.
           FileStream fs = new FileStream(TempFile,
System.IO.FileMode.Create);
           reportViewer1.ServerReport.Render("PDF", null, null, fs, out
MimeType, out FileExt);
           fs.Close();
           return TempFile;
       }

HTH,
Wayne