You can add new documents to SharePoint or you can add a link to an existing document, which exist already in SharePoint or on another location on the internet.
All you need is the URL of the document you want to link to.
The principle of SharePoint is NOT to copy existing documents (not duplication documents on multiple sites) but to link to the original document, which you also want to show on your site.
Linking to an existing document should be done by inserting a link, not a real copy.
Please note that links from Intranet to Extranet are possible, but links from Extranet to Intranet not, as the external users would not be able to open the link on Intranet.