How do I force a pdf file to save to disc rather than open in a browser window?

[snowball77]

Hi There,
I would like to force a pdf to be saved to disc rather than opening in a browser window?
Does anyone know how I can do this?
I am actually executing the command in flash in a button instance like this
on (release) {
getURL("http://website.com/pdfs/pdf_document.pdf", "_blank", "GET");
}

So how can I do this so it gives the pop up window of save to disc option?

This may be an ASP.NET issue too?