Is there any documents which briefs Whats new from the specific release. E.g I want to know the difference between Windchill 8.x and 11.x
There are separate Whats New document for every release. You can find them on PTC portal. Check following document for Windchill 11.0
You can also check update advisor to see what issues are reported and resolved between two date releases/datecodes.
To complete the previous answer, there was so many changes since 8.0 so it will be difficult to read all the what's new documents since 8.0, if the idea is to do an upgrade analysis I'd advise to use the HelpCenter for specific features to get the details on how it's covered in the new release.
Thanks a lot for the supportive info.