The fact is that you can never "upload" a Google doc. It sounds weird at first, but if you think about it it makes sense. A Google Document is always in the cloud; essentially its already and permanently uploaded. So you can't upload it again. You can only ever share a link to a Google Doc.
The solution in this case is to first download a copy of the Google Document as a PDF and then upload that PDF.
Go to File > Download as > PDF.