I have acrobat pro xi with a serial number but its not working after reinstallation on my computer. I saw that use an offline exception code can solve the problem. Can you provide with such a code and insructions?.

Hi there 

Hope you are doing well and thanks for reaching out. 

Acrobat XI is an old and EOL application and not supported on the latest Mac and Windows OS. For more information please check the help page https://helpx.adobe.com/acrobat/kb/end-of-support-acrobat-xi-reader-xi.html 

Hope this information will help.

~Amal