How to link from a PDF to Help topic?

I've seen a ton of posts on how to link from a help topic to an external PDF but not the opposite. I have a help topic that links to a PDF on the same server. The PDF displays in the same Help window. I want to place a link in the PDF that would then open a specific Help topic, but I'm not sure what the URL would be? The Help topic is already open when the user clicks the link in the PDF.

Thanks,

Charlie

charlie6067