Trying to install CS5 on OSX El-capitan. The install will not run. I have tried restarting; doesn't help

I get this error

“Install” is damaged and can’t be opened. You should eject the disk image.

“Install” is on the disk image “MasterCollection_CS5_LS1.dmg”. Safari downloaded this disk image today at 4:35 PM from trials2.adobe.com.