privacy of uploaded files

Hi,
as a PPier newby I have a question cencerning the use of
PPier for three party projects, where different clients should not be informed about the evolutions of other clients' projects.

I am trying to set up a PPier tool for the simultaneous management of several client projects, all using the same content (e.g. excel and word tools to fill in), milestones, taklists, ... provided by the owner company.
these forms ("documents"), once filled in by the different clients, should be stored in PPier, but only to be seen by the owner company and the client who uploaded the form.

It seems to me that in PPier, once a client uploads a file, it can be seen by ALL of the clients. Is this correct or am I missing something?

How can I incorporate privacy of uploaded files (between the clients)?

Is there a way to change the privacy status of uploaded documents (so that at a later moment, different clients CAN see each others work)?

Thanks,

roel