Maybe because it's PDF 4.x format? And your browser plugin don't "understand" it?
Yes, it is a problem with the browser plugin. We always write pdf 4.x as printers still prefer it.
Anyway here is another solution: Open your Acrobat Reader and disable the option "Display pdf in browser" (or similar) under preferences. Now it will always ask you if you want to open the pdf in your brower (no!) or want to save it first (yes!).