https://${sites}/_api/web/GetFolderByServerRelativeUrl('Shared%20Documents')/Files('test.xlsx')/$value

var sppull = require("sppull").sppull;

var context = {
  siteUrl: "http://contoso.sharepoint.com/subsite",
  creds: {
    username: "user@contoso.com",
    password: "_Password_"
  }
};

var options = {
  spRootFolder: "Shared%20Documents/Contracts",
  dlRootFolder: "./Downloads/Contracts"
};
sppull(context, options)....