PdfReader threw an exception

Is it possible to use both iTextSharp and iText in the same page?
I try to encrypt a document and get an exception at the line:
 com.lowagie.text.pdf.PdfReader reader = new   com.lowagie.text.pdf.PdfReader(Server.MapPath("SimpleRegistrationForm.pdf"));
The exception is: "com.lowagie.text.pdf.PdfReader threw an exception" message.

using iTextSharp.text;
using iTextSharp.text.pdf;
using com.lowagie.tools; 
using com.lowagie.text.pdf;

public void encryption(object sender, EventArgs e)
        {
            try
                com.lowagie.text.pdf.PdfReader reader = new com.lowagie.text.pdf.PdfReader(Server.MapPath("SimpleRegistrationForm.pdf"));
          &n ...