You can always download latest version of Adobe Reader 8.1 from Adobe website. However, the setup file that you downloaded is actually a online installer.

This tiny addon is fine for those who stay, live and survive online, but it cannot be used if you want to install it offline, or just for backup purposes. So it’s best to download the full installer instead, which can be used to setup and install Acrobat Reader 8.1 even when you’re not connected to Internet. The direct download links to various localized language editions of Adobe Acrobat Reader 8.1 is listed below.