If you have the exact url of your pdf file stored in an attribute, you could create a link button, with the url to your file as the address.
If the case is that your PDF is not in Mendix but you do want to download it from Mendix, you should retrieve the PDF document first. This might be possible with a web- or REST service if the other side allows this. The IFrame widget just shows an other webpage, it's not possible to get data like a pdf with it.