Install a PDF maker to the panel of a WYSIWYG editor?
I don't know of a pre-built solution for this.
I assume you want a PDF icon similar to what there is in MSWord or OpenOffice.

There is no PDF generation capability with client-side Javascript that I am aware of, so it would be difficult to implement something like this in the browser.

There are plenty of server-side PDF solutions, however, but you'd have to send the editor content back to the server somehow (AJAX), and then have the PDF generated and sent back to the client.

There looks to be some discussion of this at the CKSource forums.

It might be easier to just use the Print module.