Sorry, I'm not sure how to help you on this one, because I don't know what issue you're having. Are you not sure how to get started? Are you stuck on a particular step? Are you using an extension? Are you getting any error messages?
I'm assuming you don't know how to get started, so I'll help from that perspective. You need to upload your PDF file via your Media Manager, then you can use your WSIWYG editor's link button to link the PDF's URL.
Let me know if you have any questions and which part specifically you're stuck on.