iOS doesn't support viewing of PowerPoint presentation natively, regardless of which browser is used. Likewise, Zoomforth doesn't have a native utility to render a PowerPoint presentation.
These limitations prevent PowerPoint presentations from being viewed or rendered within the site.
As an alternative, with iOS rendering a PowerPoint presentation, an application that can read/display such files must be installed on the device. From your Zoomforth site, the PowerPoint presentation can be downloaded and opened by the application.