PDFsharp & MigraDoc Foundation

namespace Sandbox.PDFSharp
{
    public partial class Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            string filePath = @"H:\Projects\Sandbox\Sandbox\PDFSharp\template_5.pdf";

            // Open the template
            PdfDocument template = PdfReader.Open(filePath, PdfDocumentOpenMode.Modify);

            // Get the root object of all interactive form fields
            PdfAcroForm form = template.AcroForm;

            // Get all form fields of the whole document
            PdfAcroField.PdfAcroFieldCollection fields = form.Fields;

            // Get all form fields of the whole document
            string[] names = fields.Names;
            names = fields.DescendantNames;

            // Fill some value in each field
            for (int i = 0; i < names.Length; i++)
            {
                string fqName = names[i];
                PdfAcroField field = fields[fqName];

                PdfTextField txtField;


                if ((txtField = field as PdfTextField) != null)
                    txtField.Text = "Hello";
            }

            PdfDocument newDocument = new PdfDocument();

            // Save the document...
            const string filename = @"H:\Projects\Sandbox\Sandbox\PDFSharp\newDocument.pdf";
            newDocument.Save(filename);
        }
    }
}