On my Nexus 9 with Android M (aka 6), the message in subject appears when I want to open a Plate.  Any idea why it cannot open such files?  The tablet does open PDF files except those of Skydemon??  My Skydemon android version is 3.5.1.21791.

Maybe try installing another PDF viewer app?

Yes, that was it, now it works.  However, when I wanted to install a PDF viewer, it said it had already one !?  Go figure.  So I did nothing but tried the whole plate-thing again and everything works normally suddenly, PDF and all. I will never understand computers. Like the PDF viewer needed to be woken up.

Android is a little peculiar when it comes to apps that open files. It might be that your existing PDF files cannot open them in the way we need to pass them, and therefore it didn't work, but your new one can.