Thank you all for reporting! That is rather odd, I'm a bit baffled. I don't really know what could be the problem; I used a LibreOffice document with black writing on a black background, and then exported to PDF.
Could it be because of the font? I used the Caterina font; maybe if it's not installed on your computer, it won't display and/or copy?
I will investigate and release a new version of the InvisiClues soon, possibly tomorrow. Thanks for your patience!