Follow below steps to add button on any object : 

  1. Go To Setup -> Customize -> Accounts (OR any object where you want to add the button) ->Buttons and Links->New
  2. Click on New Button and Enter details
  3. Provide the following information in above screen :
  •     Label: Download Attachment (you can change it if required)
  •     Name: Download_Attachment (you can change it if required)
  •     Description : Optional
  •     Display Type: Detail Page Link or Detail Page Button
  •     Behavior: Display in existing window without sidebar or header
  •     Content Source: URL
  •     Formula Editor : /apex/DownloadFiles__MassFileDownload?parentid={!Account.Id}

  4. Finally, add the button to required page layouts of Account (or object for which this button was created) and it's ready for use.

      

          For Lightning Mode Users Add the button in Lightning Actions also, so you can use it in Lightning.