Get the XFA content as byte array.
The XFA is either a stream containing the entire XFA resource
or an array specifying individual packets that together make
up the XFA resource.
A packet is a pair of a string and stream. The string contains
the name of the XML element and the stream contains the complete
text of this XML element. Each packet represents a complete XML
element, with the exception of the first and last packet,
which specify begin and end tags for the xdp:xdp element.
[IS0 32000-1:2008: 12.7.8]