The download is for you to have a local copy of the JSDoc for the SS2 modules. You can usually import that into an IDE to have code completion and documentation available in the IDE.

In the file cabinet on the right side top corner we can see suitscript 2.0, suitscript 1.0 there when I click on suitscript 1.0, I can see some script. When I click on suitscript 2.0, Zip file is downloading automatically to local downloads in the system. Recently we approached Netsuite to upgrade suitscript from 1.0 to 2.0. They have upgraded but not activated. How I can say this means when I tried to deploy script, after creating script record we have to select type of script. There it is showing select 1.0 script type. So, please suggest me way to activate suitscript 2.0 in filecabinet. Thank you in advance.

the suitescript 2.0 zip is used while Working with IDEs Other Than SuiteCloud IDEs

selecting the type of script suggests that your script file fails the SuiteScript 2.x Entry Point Script Validation

As shown here suitscript 2.0 was enabled to file cabinet in my account but while clicking on that zip file was downloading means it was not activated right. When I tried to save 2.0 script in file cabinet am getting one pop up like failed to validate script file. This file is used by a suitscript 1.0 script. You can not change the API version of the file.

I have read entry points for suitscript 2.0 and my script consists of all of them.

again that link is related to Working with IDEs Other Than SuiteCloud IDEs

be specific about what guidelines your script file includes

its boring saying it implements all of the structures for entrypoints

there are 8 required structures, go thorugh and provide proof that you have done all 8

if you dont know what the 8 are, read Entry Point Script Validation Guidelines

minor spoiler, yo will want to go through all the related topics of SuiteScript 2.x Entry Point Script Validation, not just the guidelines topic