Normally that woudl not be a problem for PDF documents but forms are a
special case. There is script that runs at the document level (that you
do not see) and combining those forms into a single document cannot be
accomplished today because of those scripts. Normally people will
flatten the forms before they combine them ( flattening means removing
the "formness" out of the form and it is no longer interactive).One
other means woudl be to combine them into portfolio (the forms remain in
tact but...