Can not re-install CS 5.5 on old iMac

I had a legitimate copy of CS 5.5 installed on my old iMac, which I use sporadically. Not frequent enough to justify an upgrade to a paid subscription anyway.

After I had to re-install this iMacs OS (High Sierra, 10.13.6, last available for this iMac), I had to re-install the applications as well.

Adobe site, after log in, gives me my software key as well as an option to download the installation files. Nice!

After download, clicking on install.dmg opens the installation wizzard, but clicking install returns a message: “install” can not be opened. You need to remove the disk image. Same message after open with/contents/MacOS/Install.

Is there a way to open the installation file and get installation going? Obviously all files are present.

Thanks!