When you are going to install Reader/PhantomPDF on your server or deploy them to your client systems using MSI package or the command-line arguments, you may customize “set as the Default PDF viewer” in an MST file to define the MSI installer. This setting works fine if your client or server systems are based on Windows 8 or Windows server 2012 and above. However it might not work if they are Windows 7 or Windows server 2008 based. Your client user might probably receive the following UAC prompt when going to open an online PDF file
Simply and directly deploy the MSI installer package with an MST file without being assigned any default options in the Customization Tool. The Reader/Phantom MSI installer will self-register itself to be default to system and browser during the process of setup. Additionally define the default settings to the installer, might cause the above UAC prompt when the program was launched by a non-admin user。
Afterwards, be sure the “Default PDF Viewer” options in GPO template are not configured. If it is enabled, it will trigger the UAC prompt as well.
Lastly, if you ever assigned "profstore.xml" file to the program with default options, you need to recreate a "profstore.xml" file by unchecking the two default options in XML Editor and save to apply it to the program again.