Photoshop will do this for you.
However since most of us don't have photoshop, then you can use preview. Off the top of my head, a really quick and dirty way to do it would be to export it to jpeg, then convert it back to PDF.
You'll probably lose some page breaks etc, but /shrug that'll reduce the size alright.
And FYI, this is the server discussion forum. You should maybe post this here.