Hi,
Forms that are created in LC Designer (XFA Forms) can be saved as Static PDF Forms or Dynamic XML Forms. While these open in Acrobat/Reader, they are completely different from forms developed in Acrobat (AcroForms) or natice PDF documents.
You cannot edit the Static PDF (XFA) Form structure or add new fields in Acrobat. You will need to open it again in LC Designer and work on it there.
Hope that helps,
Niall