NSDocument to hold a complete folder?

Question

I ask sorry if this argument has already been covered, but after some research i found nothing precise.

I need to make a document based application, where the document is actually not a single file but a structured collection of files, in a directory. The windows will show a pdf contained in the folder, with a specific filename, and enrich it with informations from the other files in the folder. I cannot use pdf annotations to achieve this, i really need to keep the files separated from the pdf.

What's the best approach to achieve this? All the sample code i found use a single file..