John, that's a good question. Depending on the competitiveness of the keyword, I would hesitate to set up a 301 redirect--you still may see ranking changes if you replaced it with a landing page.
I suspect that the reason why it's ranking is because other sites are linking directly to the PDF file, the content. If you were to remove that PDF, they might stop linking to it.
One option would be to edit the PDF file and make that PDF the landing page (in .pdf format), that would be the same URL.