(As of March 2021, Viewer is only supported on versions of Microsoft Windows supported by Microsoft. Before March 2021, Viewer ran on Windows, Mac, and Linux. At that time, we dropped support for Mac and Linux due to low demand. This article remains for those running unsupported versions.)
OS X Gatekeeper occasionally shows the message that the application is "blocked from use because it is not from an identified developer." Click the Open Anyway button to override and run the installer.