the .7z extension is a compression format. If your system is trying to open it with Acrobat then somewhere along the line you system has tried to associate the .7z files with acrobat.
You will need to download software to un-compress the .7z file. you can get a trial version of winzip for example or perhaps winrar - both of which will read that format. (do a Google search for either of these).
During the installation of either of these you will get the opportunity to associate files with the correct program (including .7z).
Once you have done that you should be fine.