Navigation to URLs embedded in Office products must be blocked in PowerPoint Viewer.
To protect users from attacks, Internet Explorer usually does not attempt to load malformed URLs. This functionality can be controlled separately for instances of Internet Explorer spawned by Office applications (for example, if a user clicks a link in an Office document or selects a menu option that loads a web page). If Internet Explorer attempts to load a malformed URL, a security risk could occur.
Set the policy value for Computer Configuration -> Administrative Templates -> Microsoft Office 2016 (Machine) -> Security Settings -> IE Security "Navigate URL" to "Enabled" and place a check in the 'pptview.exe' check box.