Question: In a Web application, is it possible to force a PDF file to be printed on the client? If the browser is configured to open the PDF inside the window, I guess that calling window.print() will work, but some browsers (like mine) are configured to open the PDF externally.

Answer: 
you can set a http header to application/pdf and then force a new window open with javascript and print that way. but who would really do that? i mean come on now.
Do you mean that you want to force the file to be sent to a printer? Are you thinking of the Law of Unintended Consequences -- the user's device isn't connected to a printer? Could be a BlackBerry, could be a laptop on wi-fi. What if the user doesn't want it to go to the default printer?