Simply login to your website and got to the page you want to edit. Edit that page. In the editor, you will see a 'chain' link button at the top of your editor, click on this button. A window will appear - where it asks for a URL, there is a button next to it that has three dots '...'. Click on this. Then double click on the 'Link Library' link, and then double click on the 'Files' link. Here, you will see all of the files you uploaded. Double click the one you want to link to.