Not very fast...
when loading a big PDF, before the save button can be used, it needs to render the whole thing first. I don't understand why, because all it has to do is save the file which it already has.
Beside, if you know before hand that you want to save the PDF, surely you can right/Cmd click the link and save the file right away.
Anyhow, if we - Opera - decide to use PDF.js as the default PDF renderer for the Desktop browser, we will roll our own viewer which can use fancy pancy features like: <a href=# download>Save</a>