It is technically feasible to link external documents but the feature is not enabled.
As an alternative, the "add attachment to a post" is now enabled.
This feature works as follows:
1: Click on the "reply" tab of a post to which you want to reply.
2: Click on the "options" tab.
3: Click on the "add/update button at the top left of the panel.
4: Browse to the file that you want to add. (e.g. on your own desktop) and click "save"
5: Compose your post as usual.
6: Shortly after you click on the "post" button, your attachment will appear.
Please let me know if this does not meet your needs.