cogadh: Its not exactly "sneaky" when there is an option to not install it. It is installed by default, but only when it does not detect some other PDF reader on the system. A PDF reader of some kind is needed for the game manuals.
Kingoftherings: This is true, but I almost never go into the custom install options. I didn't even know there was a button for that for the longest time. I think there definitely needs to be a prompt asking if you want to install Adobe Reader rather than hiding the option in a menu that I have trouble finding.
While the PDF reader is needed for the manuals, I personally don't read the manuals. Therefore the PDF reader isn't need for the game. ;)
You trust installers implicitly and never check what they are doing? I don't care what I am installing, I never trust the "default install" options and always check for custom settings. Trusting installers is how you end up with things like the "spam toolbar of the month" installed on your system without your knowledge.
A PDF reader may not be required for the game, but in this day and age, a PDF reader is almost a pre-requisite for PC use. Given that, the fact that the installers were installing a PDF reader by default was probably not much of an issue for most people, since most of them already had Adobe or something else. As long as you have some application associated with PDF files already, the installer will never install anything extra.