To extract data from PDF to FDF, then export FDF as XFDF

doc = PDFDoc(filename)

# Extract annotations to FDF.
# Optionally use e_both to extract both forms and annotations
doc_fields = doc.FDFExtract(PDFDoc.e_annots_only) #PDFDoc.e_forms_only

# Export annotations from FDF to XFDF.
doc_fields.SaveAsXFDF(output_xfdf_filename)

# Optionally write XFDF to a string
XFDF_string = doc_fields.SaveAsXFDF()

PDF Form Fill and Form Data Extraction
Full sample code which illustrates basic FDF merge/extract functionality and full support for FDF (Forms Data Format)