Emily:
The simple answer is "no." You can replace an asset with the same filetype. So if you're trying to redirect a PDF because you have an updated version, you can replace the old version and it will keep the same URL. There is not the same kind of URL redirect functionality with assets.
While there would be a way to do this by editing your .htaccess file on your root domain, I believe (but am not entirely positive) that this isn't possible with the Marketo subdomain setup. This is also a way more technical way to try to achieve the same thing, and way beyond Marketo. That'd be something that's more for your site administrator or web developer.
Sorry the answer isn't more encouraging!
Best,
Edward Unthank
SEO/Web Specialist