The behavior is browser-dependent, and may vary with browser settings with some browsers. For me, running Firefox on Linux, I'm given the option of opening it (with a PDF viewer I select) or saving it to disk. The dialog box has a check box that tells Firefox to always take the selected action in the future, but I've never checked it. In the case of Firefox on Linux, even if you opt to view the file in a PDF viewer, it is saved to disk, but in the /tmp directory. I don't know offhand where Windows browsers are likely to save such files on a temporary basis, if indeed they do so.