You shouldn’t “guess” for the possible values. You need to use a value that is stored in the PDF. Try the CheckBoxValues example to find these possible values:
public String getCheckboxValue(String src, String name) throws IOException {
PdfReader reader = new PdfReader(SRC);
AcroFields fields = reader.getAcroFields();
// CP_1 is the name of a check box field
String[] values = fields.getAppearanceStates("IsNo");
StringBuffer sb = new StringBuffer();
for (String value : values) {
sb.append(value);
sb.append('\n');
}
return sb.toString();
}
Or take a look at the PDF using RUPS. Go to the widget annotation and look for the normal (/N) appearance (AP) states. In my example they are /Off and /Yes:
