Help With Early 2008 iMac
I need to upgrade it to El Capitan and it is running OS X Leopard so I tried to update to OS X 10.8 but it showed an error that it could not open the file (pkg)NOT saying that it was damaged
Earlier Mac models
no, you need to restore the DMG to a usb flash drive the boot from the usb stick.
Create a bootable installer for macOS - Apple Support