We've just tried to flush out our DNS using ipconfig /flushdns.

The script ran in the command prompt and said it was successful.
however, we are still seeing an old version of a file on our server.

This is how it's set up:

www.oursite/folder - this is where we take users to.
www.oursite/folder/index.html - this is where we put a redirect script
www.oursite/folder/ourPDF.pdf - this is where the redirect script takes users.

we use the same URL above every year (e.g to take users to an annual survey or to a different PDF.)
We change the .PDF file (e.g. ourPDF2.pdf ourPDF3.pdf etc), but keep the shortcut URL the same.

Now, some users (who must have visited it in the past), are still seeing the old pdf file (its cached and we can't clear it).

Is there any way we can refresh things at server level to stop this from happening?
We don't want to ask users to refresh at their end, and besides this doesn't always work anyway - as they are only refreshing the folder and not the file.

Or must we avoid using /folder at all costs and take users to the actual filetype (e.g. folder/pdfname.pdf) ?