How to return a file (FileContentResult) in ASP.NET WebAPI

In a regular MVC controller, we can output pdf with a FileContentResult.

public FileContentResult Test(TestViewModel vm)
{
    var stream = new MemoryStream();
    //... add content to the stream.

    return File(stream.GetBuffer(), "application/pdf", "test.pdf");
}

But how can we change it into an ApiController?

[HttpPost]
public IHttpActionResult Test(TestViewModel vm)
{
     //...
     return Ok(pdfOutput);
}

Here is what I’ve tried but it doesn’t seem to work.

[HttpGet]
public IHttpActionResult Test()
{
    var stream = new MemoryStream();
    //...
    var content = new StreamContent(stream);
    content.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");
    content.Headers.ContentLength = stream.GetBuffer().Length;
    return Ok(content);            
}

The returned result displayed in the browser is:

{"Headers":[{"Key":"Content-Type","Value":["application/pdf"]},{"Key":"Content-Length","Value":["152844"]}]}

And there is a similar post on SO: Returning binary file from controller in ASP.NET Web API
. It talks about output an existing file. But I could not make it work with a stream.

Any suggestions?