Adobe Acrobat X Pro

I tried Acrobat DC and hate it.  It uses too much memory, and is cumbersome to use.  I deleted it when I was able to find the download and key for my previous version (on a defunct computer), and downloaded it.  The system returned an error saying that the key was only good for an "upgrade" and since the program could not find a previous version on my new computer, it was no good.  Any suggestions or help would be GREATLY appreciated!  Thank you.