DP-G4-450GUY You pulled down the Intel version by mistake.
The PPC version runs just fine on PPC-based Macs.
You can run it on 10.5.6 - but 10.5.7 has various security updates and there have been other updates since then (from Apple) that you really should apply - Adobe is staying on the legally 'safe' side of the fence by stating they want YOU to be at the maximum security level before installing their product - protection from any liability - as every software developer will state in their system requirements. It's just using good common sense.
(Version 9.1)