res.send() express.js automatically redirect

I am using nodejs and the framework express.js to realize my website.

I am submitting a POST request on an url (/report/reportname) reportname is a variable.

so I do this :

app.post('/report/:id', function(req, res){ 
   var id=req.param('id');
   var bodyreportHtml;
   go.prototype.runReport(id,res);
  }

The thing is that in go.prototype.runReport(id,res)
I do a res.send(bodyofthereport). So when I click on submit on my form that just redirect me on /report/nameofthereport where the pdf report is. Everything is working but I would like to put this url /report/nameofthereport (the pdf report) in an iframe. So when I submit the post request I want it just refresh the iframe and print the report in it ( the iframe is on the same page than the form). But the res.send(bodyofthereport) is essential : it prints my report at /report/nameofthereport And I can't do a res.redirect('/') because the body is finished

How can I do ?

Thanks !