通过功能区代码隐藏在Word中打开文件

Using VSTO, I've created a custom tab in the Ribbon designer and added some groups and button controls there. When user clicks one of the buttons, I'd like to connect to a SharePoint site and open a word document from it in Word (an instance is already open). I'm able to connect to the SharePoint site already and have the URLs to the documents I want to open. Using VSTO, I've created a custom tab in the Ri