Hello,
You need to restructure your code so that:
http://toannn.no-ip.biz/admin/admin.export - renders the HTML page
http://toannn.no-ip.biz/admin/admin.export?pdf=1 - calls the API and returns PDF
It could look like this:
var ctx = HttpContext.Current;
if (Request.QueryString["pdf"]) {
// your current code that calls the API
} else {
// render the HTML page
}