A lo-fi way to do that could be to upload a file at Settings > Files, copy the URL produced, go to your product description and add a text link to that URL. Now, if that needs to be done for a lot of products with different PDF files for each, this might not be the most effective way, but for fewer products it should work fine.
Metafields would usually need to be employed for a proper implementation. You could create a new field where you'd either paste the hosted file's link, or upload a file directly (there are several apps that offer direct uploads). You would then need to render that metafield's information inside your product page.