Hello @Connect@Abicom,

I believe the sketch files you are referring to are from the link 

https://design-dev.pega.com/resources/download

Below are the answers (what I know ) to your questions

What is a design file ?

Our product designers uses Sketch (https://www.sketch.com/) to transform design ideas into products, every day. The design file is the outcome/design that our designer did using sketch product.

Why should I download ?

If you want to explore how our product designers did the prototypes or if you would like to modify the design for your application that you are going to build using cosmos, Instead of updating the rules in pega product, you can simply download and edit the design templates/prototypes using sketch product 

What should I do with the downloaded file in Sketch ?

Same answer of above question applies here.

If you want to explore how our product designers did the prototypes or if you would like to modify the design for your application that you are going to build using cosmos, Instead of updating the rules in pega product, you can simply download and edit the design templates/prototypes using sketch product 

How can I bring the output of sketch to pega?

This need a pega developer. I believe there is not direct way to get them (like import) into pega dev studio or any portal. There should be manual effort by pega developer who can convert the prototypes in to pega screens.

What is inside that file?

This a binary file that can understand by Sketch tool. Please install sketch tool and import these files in to that tool and see what it contains.

How it helps for working on COSMOS design in Pega?

Lets say your application requirement says to modify the existing cosmos design. The business user or user experience designers can work on this sketch design to modify and get the feedback before fully baked into the pega.

Thank you,

Regards, cherb.