Hugo,
Yes, the fallback method does work for me. However, I would be interested in what PDF reader you used that was successful in reading your InvisiClues PDF. I was unsuccessful in both macOS and Windows. I was sure it would work in Adobe Acrobat Reader for Windows.
I just played with white text on a white background in Pages and exported the file as a .docx (Word 2007 or later) file. It worked exactly as you intended the InvisiClues PDF to work.
That might be a nice third option for those of us who cannot get the PDF to work, but like the idea of selecting the text to reveal the hint. I thought that was a clever idea.