How do I install Adobe Acrobat DC on a second computer?

I have followed all of these directions, and accessed the page at Download Pro or Standard versions of Acrobat DC, 2017, XI, or X  and downloaded the Acrobat DC Pro Installer from that page and after download the program asked for my serial number (this is an install on a second computer) and I typed in my serial number for my existing Acrobat DC Pro software and the program would not accept that serial number even though it is the serial number for my active Acrobat DC Pro on my main computer (I am trying to install on another computer I use)  What is wrong with what I have done and why won't the install accept the obviously correct serial number???