This extension is useful for example when you don't know the size of the pdf file you want to view. In this case, in fact, if you load the pdf file directly in your browser you need to wait a lot of time in order to load Adobe or whatever PDF plug-in, download the file, and view the file. Using PDF Download, instead, you can see the file size in the dialog box that appears just after you clicked on a PDF link and then choose to download the file, open it or to view it as an HTML page.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.