How do I view file by format pdf on iframe

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MVC5

why when you click Submit a file is downloaded and does not display it in an iframe

What I have tried:

I wrote in cshtml:

<iframe
src="@url.action("retuenfile")" name="ifm">

And in controller:

public FileStreamResult RetuenFile ()
{
FileStream fs = new FileStream (Server.MapPath (@ "~ \ HTP.pdf"), FileMode.Open, FileAccess.Read);
return File (fs, "CV / pdf");
}