When loading the FDF document below using FDFDocument.load(InputStream) it will load fine but the returned FDFDocument instance returns null when I call fdf.getCatalog().getFDF().getFields().
In the log I get the following warnings:
Loading the same FDF file using PDFBox 1.4.0 works fine. All later versions I tried have the same problem (including 1.8.7).
The FDF document was created using Adobe's FDF Toolkit 6.0 on Windows 8.1.
You can download the FDF file from https://www.sixtyten.de/ifam/erbst_erkl_form.fdf.zip