Somehow, Acrobat Pro XI went away and I am stuck with having to licence Acrobat DC. How to get back to previous Acrobat Pro XI?

after securing your serial number, uninstall acrobat dc using the uninstaller and uninstall acrobat xi using the uninstaller.

then clean if windows per, Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs  and restart your computer.

reinstall acrobat xi and activate with your serial number.

Downloadable installation files available:

• Suites and Programs:    CC 2017 |  CC 2015.5 | CC 2015 | CC 2014 | CC | CS6 | CS5.5 | CS5 | CS4, CS4 Web Standard | CS3
• Acrobat:  DC, XI, X | X Suite | 9,8 | 9 pro extended exe,9 pro extended 7z | 9 standard  | 8 standard |
• Authorware:  7.01 windows |
• Captivate:  9 | 8 | 7 | 6 | 5.5, 5 | 1
• Cold Fusion:  11, 10 | 9.0.2, 9.0.1, 9.0.0, 8.0.1, 8.0.0, 7.0, 6.1, 6.0, 5.0, 4.0 |
• Contribute:  CS5 | CS4, CS3 | 3,2
• FrameMaker:  12, 11, 10, 9, 8, 7.2 |
• Lightroom:    6.0| 5.7.1|  4.4 | 3.6 |
• Photoshop Elements:  15,14,13,12,11,10,9,8 | 9,8,7 win | 8 mac | 7 mac
• Premiere Elements:  15,14,13,12,11,10,9,8| 9, 8, 7 win | 8 mac | 7 mac

Download and installation help for Adobe links

Download and installation help for Prodesigntools links are listed on most linked pages.  They are critical; especially steps 1, 2 and 3.  If you click a link that does not have those steps listed, open a second window using the Lightroom 3 link to see those 'Important Instructions'.