The GSX manual opens just fine with Acrobat DC. It's not clear from your description, if you cannot open it from the "Open Manual" button in the Addon Manager, or if you cannot open it even if you browse the FSX\couatl\fsdreamteam\GSX folder and open it manually.
If you just can't open it from the Addon Manager, but can by browsing its location, it's not a problem of the file, it's just that your Acrobat is not correctly registered to "start" a .PDF file. Technically, the operation made by opening the file from the Addon Manager button, is not really "opening", but it's "starting" a .PDF, which requires the related application to be properly registered to do so.