Steps
1. At first download iTextsharp
2. Then add iTextsharp.dll file as reference of your ASP.Net page
Now
You have to use following lines in your aspx page.
using iTextSharp.text;
using iTextSharp.text.pdf;
Now use the following code....
Document firstDocument = new Document(PageSize.A4.Rotate());
PdfWriter.GetInstance(firstDocument, new FileStream("D:\\Gopesh.pdf", FileMode.Create));
firstDocument.Open();
firstDocument.Add(new Paragraph("First Pdf File made by Gopesh using iText"));
firstDocument.Close();