Manuals won't open with PDF Reader.
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I don't know why you can't open those files, but as a workaround, since you can open them Google (I assume you mean Chrome), you can print them from the browser but instead of choosing a real printer you can "print" them as a pdf document. Maybe that will create a pdf format that works with your reader.
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I've never heard of such a thing, but if you make a ticket with Just Flight support, it's possible that they've seen something like this before. They have much more experience troubleshooting general issues than I do, while I'm here just to specialize in my aircraft.