I am a developer of a commercial software product that could benefit from dopdf.
Can I include a copy of the dopdf installer, unaltered, my product's cd?
You are not allowed to sell dopdf as a separate program or charge users for downloading/using dopdf.
However in your case it seems that you want to distribute dopdf installer on the cd where your commercial product is placed too, so yes you are allowed to distribute it as long as you don't alter the installer in any way.
Additionally you might want to include on your website a link to dopdf where your users could get the updated version. Again, this is optional not mandatory. You can share here the name of the application if you want ( or via email@example.com ) if it's not something you want to keep private.