Add a button to document library in Sharepoint online

vaibhav_vashist If you are using SharePoint online, you can achieve this using SPFx ListView Command Set extension.

For more information on how to develop it, check this documentation: Build your first ListView Command Set extension 

Please click Mark as Best Response & Like if my post helped you to solve your issue. This will help others to find the correct solution easily. It also closes the item. If the post was useful in other ways, please consider giving it Like.