Thank you for the quick reply. 

For Clarity....

I should create an OnPreHTTP( ) method that tests my parameter and set the ContentType accordingly.

Pseudo code...

OnPreHTTP()  as %Boolean 

{   

    set pdfToken=$get(%request.Data("pdfToken",1))
    if (pdfToken  is valid)   set      set %response.ContentType="application/pdf"

    else  set %response.ContentType="text/plain"

    quit 1

}