Navigate to the Reference section and ensure that the page editing option is enabled, when on the spanner will look like the image below
Navigate to the page you wish to add or alter the file on in the tree view, once located hover over the spanner icon to the right of the line to reveal the edit icon (it will in effect replace the spanner icon) as shown below
This will open a new window with the page on it, if you are adding a new document simply click the cursor to the location you want the link to appear, if you are replacing an existing file or link highlight the existing entry and delete it, ensure that the cursor remains where you want the link to appear then click the upload document icon as shown below (it's the PDF icon on the top row).
On the new window that pops up, select the upload button then use the standard windows interface to browse to the location of the file, selecting it and clicking open
The "Url" field should self-populate with the files new location and the title will try pick a meaningful name but usually needs correcting, the title is what will appear to users as the link to click on the page so make sure this is understandable and relevant to the user.
Once the details are correct (the properties section can be ignored on the left) click the Insert button to make the link appear on the page
This will close the window above and take you back to the edit page window. If there are no further changes to make click the save icon at the top left of the screen to save the changes and close the edit window. It is likely (depending on the browser you are using) that the page is still showing the old details press CTRL+F5 to force it to reload and it should appear with the new content. If it does not then you will need to clear your browser cache to make it get the new page, these instructions can be forwarded to you at your request.
(it will in effect replace the spanner icon) as shown below
