Highlight your text or select the image you want to link and choose the "Website Links" button. Once your text is highlighted, Select the "Website Links" category in the middle of the page.

Click the "Document Library" option. 

If the document needs to be uploaded select the "Upload Documents" button. If the document has already been uploaded, click the Insert Link button. 

Select the Choose File button and locate the file that needs to be uploaded. This places the file into the uploader. Choose the "Upload" button to upload the document to the Document Library.

After the document is uploaded, locate it from the list provided and choose the "Insert Link" button.

The text or image is now linked to the file you uploaded. Be sure to save your changes when you are finished editing the page.