How to convert PDF to text file in iTextSharp

I have to retrieve text from PDF file. But using the following code I only get empty text file.

for (int i = 0; i < n; i++)
{
    pagenumber = i + 1;
    filename = pagenumber.ToString();
    while (filename.Length < digits) filename = "0" + filename;
    filename = "_" + filename;
    filename = splitFile + name + filename;
    // step 1: creation of a document-object
    document = new Document(reader.GetPageSizeWithRotation(pagenumber));
    // step 2: we create a writer that listens to the document
    PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(filename + ".pdf", FileMode.Create));

    // step 3: we open the document
    document.Open();

    PdfContentByte cb = writer.DirectContent;
    PdfImportedPage page = writer.GetImportedPage(reader, pagenumber);
    int rotation = reader.GetPageRotation(pagenumber);
    if (rotation == 90 || rotation == 270)
    {
        cb.AddTemplate(page, 0, -1f, 1f, 0, 0, reader.GetPageSizeWithRotation(pagenumber).Height);
    }
    else
    {
        cb.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);
    }
    // step 5: we close the document

    document.Close();
    PDFParser parser = new PDFParser();
    parser.ExtractText(filename + ".pdf", filename + ".txt");
}

What am I doing wrong and how should I extract text from PDF?