In your project information workflow, for each document you'll want:
a record copy: an exact copy what you received or sent that should never be changed so it can be definitively referred back to.
a working copy: a copy of the document that you modify and mark up.
With other PIM systems such as Newforma, both the record copy and the working copies are stored on your local network, and you need to be extremely careful not to edit or delete the "record copy".
When using TonicDM, a record copy of everything sent and received is automatically saved in TonicDM and is "immutable" in that it can never be changed.
You can download working copies and organize them on your local network to modify or mark up, but you don't need to be concerned about maintaining the record copy yourself, since it will always be available in TonicDM.