I logged in to the Adobe website, followed directions carefully, downloaded first AdbeRdr 940, then AdbeRdr 8, finally AdbeRdr Upd 941. Each time the installation was refused ("not supported"). Which version of Reader do I need? How can I find out what is already installed? (My iMac was a hand-me-down from a co-worker).
Knowing the OS is important, but so knowing the hardware. Do you have a PPC or Intel-based computer. Modern versions of reader won't work on PPC hardware.
Try this link: http://get.adobe.com/reader/otherversions/