Хорошо, я нашел решение
public static void scanFields(String path) throws IOException {
PdfReader pdfReader = new PdfReader(path);
AcroFields acroFields = pdfReader.getAcroFields();
HashMap<String,AcroFields.Item> fields = acroFields.getFields();
Set<Entry<String, Item>> entrySet = fields.entrySet();
for (Entry<String, Item> entry : entrySet) {
String key = entry.getKey();
}
}