First, upload the file to your site. This can be done from the admin panel. Navigate to Communication->Web Site->File Upload. Click "Choose File", select a file from your local file system, and click "Upload".
Once the file is uploaded, click the link that corresponds to the uploaded file. On the section labeled "Currently Selected File", you will find the URL to the file. This can be copied and pasted into a text widget of the web editor.
Tags: admin, file upload, public site