Click CMS > Manage Pages or Manage Posts (depending on what page you wish to include the link).
Click the yellow edit pencil icon alongside the page/post you wish to edit.
In the content area write some text for the link and where you would like it to appear (eg: Click HERE to download).
Now highlight this text and click the link tool and click "Insert link".
In the pop up window either find your previously uploaded file OR click Choose file or Browse (depending on your browser) from the top and find the file on your computer and click submit.
Once successfully submitted scroll down through your website folders and click the file you wish to link. The URL field will automatically populate.
Click Insert at the bottom of the page.
Now save your Page/Post changes.