If you are a designer sending a file to a client using the Free Viewer, always use the File > Archive function in your full version before sending. This packages all external symbols and padstacks into the .brd file, ensuring the viewer can render the design without access to your local libraries.
You cannot simply find a standalone "Viewer.exe" on a generic download site. You must visit the portal (support.cadence.com) or the OrCAD/Allegro download page. allegro free viewer 17.4