Windows Operating System Issue

I think everyone in this thread has contributed to the best of our knowledge with your particular inquiry.

In my opinion, now that you've explained your situation more clearer, I thought that you meant that you performed an upgrade because you wanted it.

What probably happened was that the Acrobat Pro XI  was never deactivated or uninstalled to begin with. You probably just went ahead and let windows wipe everything off like most windows users do.

I provided all of this information for you to read in the links above.

You just have to make some time to read and sift through it  in order to  understand which steps of the deactivation process you could've missed; maybe then, and only then,  you can build your case to rebuttle your concerns with Adobe customer care.

You still need your license key or serial number and proof of purchase.

Saying that you bought Acrobat Pro XI  online is not sufficient. You'll need to contact that entity that you said you paid for to download that product and see if this software is legitimate or not. This was already answered and addressed by  Test_Screen_Name.

To back up what Test_Screen_Name observed, Your issue seems to be clearly explained in the paragraph below (I quote from an online article that I am reading): 

"Adobe allows you to transfer your copy of Acrobat to any computer in your business, as long as you also transfer your license and activation. If you don't have an installation CD you can download the software to the new computer, but only if you purchased Acrobat directly from Adobe. "

That said, was your Windows 7 a genuinely validated operating system when you purchased it?

Back in the day MS Windows 7 and Windows XP were not free operating systems (not saying that MS Windows 10 is free either, but  Micrososft did offer the migration to Windows 10 for free, specifically for MS Windows 7 user as part of a security migration strategy that they announced when MS Windows 7 was reaching its End of Life support cycle). 

In addition,  Micrososft used to provide a tool to validate if the licensing of such operating system was genuine or not: Windows Genuine Advantage validation tool .  Did you ever get error messages about your Windows 7 software?

Let's say that you find out that your current operating system and Acrobat software are not compliant; you're in a dead end street. You will get no support unless you purchase the legitimate software. 

So if you did purchased  a legitimate subscription of Adobe Acrobat Pro DC (not the classic track that you claim you've purchased in the past from an unkown online vendor), then you should be able to use Adobe Acrobat Pro DC with no problem..