This often indicates a modified DMG, sometimes including a "crack" or, more commonly in recent years, a patch that allows the app to bypass app-store validation or run on incompatible macOS versions (like macOS Mojave, Catalina, or newer).
Modern macOS security (Gatekeeper) will likely block the installation of a modified DMG, throwing "damaged file" or "unidentified developer" errors. Apple Aperture 3.6 -ked-.dmg