It could be a bug or the material could be redacted or the material could be covered with boxes. Can you post the file for us to look at?
The file should be absolutely ok.
When I just open one pdf document, there is no problem.
But when I open two, then the problem comes up, but only one of the document have the problem, the other did not
I have try update from v9.0 to X, uninstall and reinstall, the problem is there all the same