using (var clientContext = new ClientContext("https://tenant.sharepoint.com/"))
     {
         clientContext.Credentials = new Microsoft.SharePoint.Client.SharePointOnlineCredentials(userName, securePassword);
         Web web = clientContext.Web;
         clientContext.Load(web);
         clientContext.ExecuteQuery();
 
         List docList = web.Lists.GetByTitle("Documents");
         ListItem docItem = docList.GetItemById(1);
         clientContext.Load(docItem, a => a.File);
         clientContext.ExecuteQuery();
         FileInformation fi = Microsoft.SharePoint.Client.File.OpenBinaryDirect(clientContext, docItem.File.ServerRelativeUrl.ToString());
         var fileStream = fi.Stream;
         using (FileStream outputFileStream = new FileStream("C:\\Test.jpg", FileMode.Create))
         {
             fileStream.CopyTo(outputFileStream);
         }

     }