Thanks for posting this question. Unfortunately, we don't have granular control over PDF indexing. As I understand, the only way to change that is by changing the HTTP headers for the file.
I can offer a few potential workarounds:
Third-party Hosted PDF
You could host your PDF somewhere like Google Drive or Scribd, where they allow for unlisted documents, and then link to the PDF in Prismic. You could even set up an Integration Field so that all of your PDFs are listed in the Prismic Writing Room.
You could host the file yourself and set the HTTP headers to prevent indexing. Like with third-party hosting, you could set up an Integration Field to index your PDFs. This option would be technically advanced.
Alternative File Formats
In my experience, Google aggressively ranks PDFs, because they are often high-value content. You could try another file format that Google doesn't index, like ePub or MOBI. You also might be able to use an infographic-style PNG, and block Google from indexing it. That way you could host the document in Prismic.
I hope one of these solutions might work for you! Let me know if you have any questions.