PdfDocument document = PdfReader.Open(@"D:\Testzeugs\Test_doc.pdf", PdfDocumentOpenMode.Modify);

        PdfTextField currentField = (PdfTextField)(document.AcroForm.Fields["SOMEFIELD"]);

        //This section makes the text visible
        if (document.AcroForm.Elements.ContainsKey("/NeedAppearances"))
        {
          document.AcroForm.Elements["/NeedAppearances"] = new PdfBoolean(true);
        }
        else
        {
          document.AcroForm.Elements.Add("/NeedAppearances", new PdfBoolean(true));
        }

        //test
        string caseName = "Hallo Welt";
        PdfString caseNamePdfStr = new PdfString(caseName);
        currentField.Font = new XFont("Arial", 8);

        //set the value of this field
        currentField.Value = caseNamePdfStr;

        // Save the document...
        document.Save(@"D:\Testzeugs\test.pdf");