Peat - you can't attach a file to a comment (topic reply), but try adding a new topic (like this one). Below the post editor, there are a bunch of options. The one you're looking for is "File attachments". There you can attach your pdf.
Alternatively, you create a page / blog entry / book page / story via "Create content", and only refer to it on the forum.
Please ask these questions on here, OK?