When trying to view/download the Gift Aid declaration on mobile, it is cut off in both landscape and portrait mode. On desktop, the declaration is downloaded when you click the button, but it is difficult to download on mobile.
The workaround on iPhone is to click the Download Declaration Form link. On the new "page" press and hold the screen until a dialog box comes up. Press the "Open Image in New Tab" choice. You should be able to view the declaration.