First click the orange button to edit either the page or post you want the link to the file to appear.
Now in the content area write some text for the link (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 the folder you want to upload the file to.
Click Choose file or Browse (depending on your browser) from the top and find the fil on your computer and click submit.
Once the file has uploaded click the file icon to highlight it.
Click insert at the bottom of the page.
Now save your changes.