How to regularly update a link to a file?

My apologies; I’m not exactly sure how to explain this!

I’m building a wordpress site for a client. There’s a document (a pdf, actually) that one of their people updates once a month. (They create/update the document on their computer, externally to wordpress.)

We don’t want the actual document contents on the site; we just want a link to the document on one of the pages.

What’s the best way for the author of the document to get that document to the same spot in the file structure of WordPress every month? We can overwrite the previous month’s document.

I’m not to keen on giving this person a login, as I don’t know if I can give them access to *just* that functionality. I’m also thinking that if I give them access and they “Add New” to the media library, the link will be different, thus breaking the link on the page. FTP isn’t really an option, as the author changes once a year, and I don’t really want to teach somebody else how to do this every year.

I’m fairly unfamiliar with WordPress in general, but I’m sure there must be a way to do this. If there’s a completely different way, I’m game! I just needs to be quick and easy. (I’d love to give the author an email address and have them just email it to a specific spot on the site…is that possible?)

Thanks for any help!