There is a Microsoft Office bug (2019105) that will attempt WebDAV windows authentication from non WebDAV links. To work around this bug the server can specify the "Content-Disposition" header as an "attachment" (requesting the browser save the file instead of opening it). Microsoft Internet Explorer will still ask if you want to open the file but will not attempt WebDAV. When making links with ShowItemData.po use the "download" in place of the "filename" parameter. For example:

/ShowItemData.po?handle=123&download=file.ext