Unfortunately, installation of Adobe Reader on Nokia E6 is a bit complicated. I have read a discussion post on the Microsoft forums related to Nokia E6 and users complained about the issues they had with Adobe Reader. I recommend checking the discussion post to read about it.
However, to install it, scroll down to the last post and download the SIS file. Transfer it to your phone and install it. The file you have to download is amr100_NOC.sis.