As far as I am aware you can't do this through Tableau.
There's nothing stopping you from putting the file on a shared folder on the same box as your server and then referencing that via html, although if you are publishing I'd guess you might have other procedures in place to publish documents and .pdf files from a downloads area - this would be more efficient and repeatable.
If you are using Tableau Server 10.4, you can certify Data Sources - which allows you to add a note about the data source for your users:
I think this might meet your needs?
Thank you Carisa!
I'll look into this option. By the way, do you know if there is character limit to the notes.