I need to purchase Adobe Acrobat XI Pro...

Adobe no longer sells licenses for older versions of Acrobat. The current version of Acrobat is Acrobat DC, available as either a perpetual license or as a subscription.

Is there a reason why you need an old version of Acrobat?

Note that although you might find older versions of Acrobat (or other Adobe applications) on sale vi eBay or other on-line reseller marketplaces, be aware that much that is being offered is not from authorized Adobe resellers and is often pirated or otherwise illegal software. Buyer beware!!!

          - Dov