I just went through each of the pdf documents associated with the quizzes in Topics 1 through 4. All of the quizzes are downloading correctly and opening with no issues. Please make sure that your pdf reader is up to date and that you are using the most current version. We recommend Foxit Reader if you are having issues with your current pdf reader. I also commented on another document that you had said you couldn't open so it seems to be an issue on your end, most likely with your pdf reader.
I just went through all of the quiz PDFs once again, and each of them are downloading and opening with no issues. Please see attached images to show that after clicking the download button, they are downloading (at the bottom of my screen). Please check to make sure that this is not a browser issue by trying to open the PDFs in a different browser. As suggested earlier, you will also need to make sure that your PDF reader is up to date and is the latest version. I also provided the link for Foxit PDF reader, which we highly recommend.
Thanks for checking. For good order, foxit would help because the issue was not on the reading, but on the downloading. Basically, there was no PDF to open, because no PDF was downloadable. When I pressed the download pbar I directly got the error message.