How do I sign docs within Box?

Checkout Box Sign where you can sign and send docs to others for their signature. 

https://support.box.com/hc/en-us/articles/4404086827411-Introducing-Box-Sign

Box Sign enables you to send your important documents in Box to people for signatures. Whether you're sending contracts, waivers, proposals, enrollment forms, or legal forms, Box Sign enables you to seamlessly and easily collect electronic signatures on shared critical content, and keep your important business processes smoothly flowing.