Please note that according to our EULA only one free license key is allowed per individual, company or organization. In this case, you can use the same license key that was used to register your Viewer before. In addition, please also note that Viewer for Linux did not require registration prior to September 2021 as it was available with free access period due to Viewer for Mac/Linux being in Beta stages.
This September we've added a licensing support, so that the licensing in the latest version works the same way as in Viewer for Windows, i.e. you can use Viewer for Mac/Linux in trial mode for 30 days after which you need to register your Viewer with your license key. In order to register your Viewer for Linux please try opening the License Key Storage by navigating to the Help section -> License Key Storage on the Viewer's toolbar. In the opened key storage window you can register your Viewer by drag-and-dropping your license key to the storage window or by locating the license key file manually. After registering Viewer it should store your license key in the configuration file, so there's no need to register it monthly.