// Import form field data from XFDF to FDF
FDFDoc fdf_doc = new FDFDoc(FDFDoc.CreateFromXFDF(xfdf_filename));
fdf_doc.Save(output_fdf_filename);

// Optionally read XFDF from a string
FDFDoc fdf_doc_string = new FDFDoc(FDFDoc.CreateFromXFDF(xfdf_string));

// Merge FDF data into PDF doc
PDFDoc doc = new PDFDoc(filename);
doc.FDFMerge(fdf_doc);