Put break-points in your code: when you hit the break-points single-step through your code and examine the values of the variables. Use F11 to single-step in Visual Studio.
Either you are over-writing the file, or you are not accessing the changed content in the TextBox. This should be simple to figure out.
if (pdfFile.ShowDialog() == System.Windows.Forms.DialogResult.OK)
{
Document document = new Document(iTextSharp.text.PageSize.A4, 40, 40, 50, 50);
PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(pdfFile.FileName, FileMode.Create));
document.Open();
document.NewPage();
ParagraphPDF title = new ParagraphPDF("AAA", titleFont);
title.Alignment = Element.ALIGN_CENTER;
document.Add(title);
document.Add(Chunk.NEWLINE);
ParagraphPDF p1 = new ParagraphPDF(textFromTextbox, infoFont);
document.Add(p1);
document.Close();
}
Also, put a break-point just after this code returns.