DocInfo is a tag set that is part of the DDX command file. LiveCycle Assembler uses a DDX file to determine what it is supposed to do to a file. Have a look at the DDX reference guide: http://livedocs.adobe.com/livecycle/8.2/ddxRef/001000.html
and specifically at the Doc Info part:
(You can see the table of contents from the above links by clicking on the "Show Navigation" icon in the upper left)
In workbench I use a SetValue
Before you assemble the document you need to build the DDX string and specify you want the xml info.
concat(/process_data/@ddxHeadStr, ' <DocumentInformation source="mergedPDF" result="doc_info.xml" />')
Use SetValue set your xmlinfo variable (type xml) to this
/process_data/xmlInfo Set TO
Then get your pages from the XMLinfo
Hope this helps. Email if not clear. I had to fight through this recently and feel the documentation is really lacking.