Question: pdf rendering as black rectangle
Pdf's are rendering as black rectangles in Preview.app. Notice the thumbnails in the screenshot. They also render thusly if I tap the spacebar in finder for a Quick Look. I have tested 20 or so pdf's. I have not tested all of the pdf's on this system, obviously (far too many). Opening the exact same file in, e.g., Firefox displays it properly.
Mac mini, macOS High Sierra (10.13)