Saving PDF files on multisite installation


I'm writing a simple plugin which needs to save PDF files in a multi-site installation.
I need to define where to save the files.

The first option which comes in my mind is creating a subfolder in the upload directory.
Making it work on multisite would simply require to define the path as wp_get_upload_dir().'subfolder'
In such a way each site of the network would have its pdf directory

But doing that would mean seeing the PDF files in the "Media" section of the admin panel, which is something I don't want, as I'm creating a simple app to view the PDFs.

So, can you help me to define where and how to save the PDFs ?

Thanks for your help

Kind regards