XML file created in the XML Data Package format, a file format developed by Adobe Systems for packaging PDF data into XML files; contains the entire PDF document contents including form and template data; may also include character-encoded sections for binary content.
The XDP format allows a PDF file to be transformed to XML and back to a PDF without losing information. It also enables any XML program to read the PDF data.
Adaptive Forms or Experience Manager Forms from Adobe can store and transmit online form data to multiple sources. Often times, a PDF document of record (DOR) is needed. You could use AEM's HTML/CSS based option to generate PDF DORs or more traditionally use XDP format files to generate PDF based DORs. The drawback of using XDP based DORs in AEM is that you will need an additional skill set of Adobe Designer tool.