I was trying to send a pdf file to one of our subcontractor but the pdf file is too big and needs to be reduced, while searching the net, I stumble upon one great online free services that actually do this thing and a lot more, with very clean interfacing, I'm sure you will be adding this to your favorites.
Smallpdf.com - as the name implies it really makes a pdf file smaller. A browser-based and work for all platforms, it does not require any download or registration. All you need to do is to drag and drop your pdf file in the box, wait for the compression to finish and download the file, you can also save it to your dropbox.
Aside from compressing your pdf file, it has a lot of services like converting your file into another application like word to pdf, pdf to jpeg or merge your document into one single file.