void addDocumentField(PdfIndirectReference ref) {
        PdfDictionary catalog = reader.getCatalog();
        PdfDictionary acroForm = (PdfDictionary)PdfReader.getPdfObject(catalog.get(PdfName.ACROFORM), catalog);
        if (acroForm == null) {
            acroForm = new PdfDictionary();
            catalog.put(PdfName.ACROFORM, acroForm);
            markUsed(catalog);
        }
        PdfArray fields = (PdfArray)PdfReader.getPdfObject(acroForm.get(PdfName.FIELDS), acroForm);
        if (fields == null) {
            fields = new PdfArray();
            acroForm.put(PdfName.FIELDS, fields);
            markUsed(acroForm);
        }
        if (!acroForm.contains(PdfName.DA)) {
            acroForm.put(PdfName.DA, new PdfString("/Helv 0 Tf 0 g "));
            markUsed(acroForm);
        }
        fields.add(ref);
        markUsed(fields);
    }