Documents can be uploaded from within the editor widget when editing a page.
To add a link to a document you have uploaded to the website (or to upload a new document), click the point in the text you want the link to appear and then click the document manager icon on the toolbar of the editor, and you will see a window popup with a list of documents added (and the option to add new documents). To upload a new document click the upload button, which will allow the upload of documents from your computer's hard drive to your website.
To create the link just click the document you wish to include. This will create a link which by default will use the actual location of the document as the text for the link. To do this right click the text added, and you will see an option "set link properties" click this and then change the value next to the link text heading. Your link will now be set to open your document but will display a meaningful name.