This could be a difficult question to answer, depending on the approaches you have attempted.
Please provide the code so that it can be studied.
(Not a complete codedump!, Just the relevent area
There should be no readily apparent reason why a PDF is not opening, (Unless perhaps there is no PDF reader loaded? I presume this is not the case)
Another approach is to not load a PDF at all, and let the user choose to save once he has loaded it to his browser (This is an approach I favour with my company's website).
Either way, code would allow us to provide a better, more suitable answer for you.
Mark up if helpful!