Some others are saying: "Let's not give so much confidence to the average user's ability to punch that back button in the browser bar. Let's not assume the casual visitor will much care if he's landing on your site or on some other web site". In fact, thinking from a balanced perspective, what makes your web site stand apart from other sites? In the mind of the web user, I mean. It's more probable that random users will treat your site equally in respect to others. And then you need to "help" them notice your site "more than otherwise" noticed. Therefore it makes sense to mistrust the urge and sometimes capacity of the average users punching their browsers' back buttons only to return back to your beloved web site.
For that matter:
1. Whenever you're about to code an external link out of your web site, then make sure it's targeting a new (_blank) window.
2. Whenever you're about to serve some non-HTML content to the user, such as a PDF file, then do that with a target to a new window again. Why so? Because a PDF will call to open an Adobe application in your browser, or along your browser. That's not just loosing a visitor from your web site alone, but sometimes even hiding the entire browser application beneath the Acrobat from Adobe, which is handling the PDF.
3. On a secondary note, menu links (and especially sub-menus) are nicer when opening in the parent window. Hence not a very inspired idea to outlink to PDFs directly from the main menu of your web site. And even worse to have those PDFs open in the very parent window to replace your site with another application, from Adobe...
If you're familiar with Google Apps, then you'll be amazed to notice how many and how often you land on new windows (or tabs if your browser is set to open tabs instead of windows) whenever you give a click on a top outer link. Like from Gmail to Documents, or to Calendar and so on. Seems that Google understands to take care of the users needs by offering them more than one single option: that named browser back button to punch on.
