thanks for the precise explanation.
But this happens to me to what I believe are regular PDF files. I'm on a Mac and certain PDF files are treated that way. Those PDF files do reside in regular folders and are not being edited or something. Needless to say, the PDF's are opening fine both in Mac preview and Adobe, so the files are good.
How exactly do you detect a file being a temporary file on a Mac? I don't see anything special in the properties of the affected files when comparing them with non-affected files.