Parallel multi-instantiation with a list of documents

I have a process that includes iteration Parallel multi-instantiation task that will send a task to different users.
For now, I only can have simple inputs with text, boolean values, etc, but my manager wants me to put document file field so that the users can upload a file if they want.

The document cannot be added in the parallel multi-instantiation output results, so I have to figure out how to manage the separate multiple document files in a different data variable. I am not sure how.

Can anyone give me an advice on this?