Can you please let us know if this license is free, or do we need to purchase it for commercial use?
The plug-in viewer is free to download and the installation too, but its usage is nowhere defined on the official website. The only thing available for you to try is the installation of the plug-in on one of the supported platforms. Access the link below then click on the downloads and select the version for your operating system. Proceed with the installation, then simply access it. You will be notified if it's free or requires a certain license to run properly.
Note: I've also downloaded the user guide and it's nowhere specified about the license, therefore it appears to be free.