As far as Windows 10 is concerned, I think this is just an OS Version/Build issue. I experienced the same problem on my Windows OS generally and Initially I thought it was a Windows OS non-compatibility issue with Odoo, but my assumption changed when I got a new PC with a very recent Windows 10 Pro Edition and when I launched Odoo on it the negative sign was coming out correctly in all PDF reports.
However, another PC with Windows 10 Home Edition was still having the issue but when I updated that one too to the latest Home Edition the error vanished completely.
So in a nut shell if your system is Windows 10 simply update your OS and that should resolve the problem.

I am yet to test the effect of the OS update on Windows 7 and any Windows Server.