Hi Bram,
it's not the extension that tells the browser the nature of the page contents, it's the header field called Content-Type:
When PDF reports are generated and sent to the browser, the Content-Type is set to application/pdf.
On the browser side, the browser then checks to see what apps it has listed for that application type, and opens one as appropiate.
So the most likely reason for it not opening is that there's not PDF reader installed on the client machine.
Cheers
Bruce