Select element that will be used to download a file (example: image, button, text);
Find the "URL" radio button in the right toolbar's first tab;
Click on the "Pencil" button by the "No link selected" option, you will see a "Link properties" popup;
Choose the "File from your Website" radio button and click on the "Pencil" button;
Click on the "Upload New File" – upload the necessary file, click the "Select" and after “Apply” button.