Link to a FTP VS link to a LMS

I am not sure why you would have to put the target PDF in the LMS unless you need to associate some actionable task with that (tracking downloads, perhaps). In our organization, we simply point the link to the PDF to an internal site where all employees can access it. In the Storyline file, we then put that link directly in the file so it's available either as a Resource or as a clickable item.

So in your case (a button to allow users to download a PDF), you would create the button, copy the link to the PDF, and then paste that link into a hyperlink trigger for the button.