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.

I have the same issue with the negative vertical bar ('|'). It used to work fine until now. Please help us fixing this issue. It must be a simple fix.

Thanks

I have the same issue (V9) but I do not have the custom module account_financial_report_horizontal 

Though, it prints -800.00 on the tax line of invoices when I was testing/playing around. Due payments and almost all other reports I have tested have the vertical bar issue.

I guess we can conclude its not THAT custom module. Maybe some other one? Or something else?

ADDED INFO: I tried removing all custom modules that generate a report and no change.

UPDATE 23/05/2017: I had this issue on my Win10 localhost. When transferring to a production Ubuntu server the error vanished.

Cheers.

Does any know which module is responsible for reports?

I will look into that module xml code to try to find solution.

Thanks