Right-click on any document already recorded in PaperOffice and select "Connect with a contact".
Select an existing contact from the list or click on "Create a new contact" and create a new contact.
Now click on the green button "Connect selected contact to the current document" - now the document is connected with the contact you have just selected. You can connect multiple contacts with the same document by repeating the above steps.
To disconnect, select the document in the document list and call the context menu, select the item "Disconnect from a contact". Now click on the orange button "Disconnect from current document" - the connection has been disconnected.