From your Practice Documents

Your Practice Documents are found under Admin > Practice Documents. You can save incoming faxes to your Practice Documents using the Add to Office Documents option to the right when viewing an incoming fax. You can upload documents that are saved on your computer to your Practice Documents by clicking on the plus sign at the upper right corner of the Practice Documents window, then browsing your computer for the document you want to upload.

Fax a document from your Practice Documents

Right-click on the document name and select Fax this Document.

A new tab will open up with an editable cover page (click on the page to edit or add text).
The document itself is not shown in the faxing window, but it will be transferred along with the cover page. Type in the fax number, and click Send as Fax to send.

From a Dummy "Patient" Chart

Another option is to create a dummy "patient" account called Practice Fax or similar, and save or upload and fax documents from that "patient's" chart.