Hi @prismic_user,
Thanks for posting, and welcome to the Prismic Community 
This problem is actually about to be partly resolved.
We've received a lot of feedback from users that they have trouble managing PDFs in search engine results, so we have decided to prevent PDFs from appearing in search engines. This gives users more control over how their content is distributed. It also improves security. (While we discourage users from uploading sensitive information to the Media Library, sometimes it still happens, and that information can get unknowingly indexed and distributed in search engines.)
We have already changed our search engine settings to block Google from indexing PDFs hosted with Prismic. The change should take effect over the coming weeks.
As per redirecting a PDF, I don't think that's possible. (I'm checking with my dev team to make sure, and I'll let you know what they say.) If you have one file that you need to be able to update or replace regularly, it might be better to host the PDF on your own server, so you can have absolute control over it.
I hope this clarifies things. Let me know if you have any other questions.
Best,
Sam