Document management with child elements

Hello,

For a several hours now I’ve been searching for a document management plugin which allows to add child elements to parent documents, but with no success.

A functionality, whitch is quite important to me should work like this:
I’m adding a file, let’s call it “main.pdf”, and then add a “secondary.pdf” which is chil element of the “main.pdf”. Hope that what I wrote is understandable:P

So, to sum up, I would really appreciate some hel or suggestions.