The PDFs are encrypted. They won't render in macOS Mojave Preview; I can enter the password but I see a blank page. (For more on all the many issues with PDF on every platform but especially macOS see the great Eclectic Light series on macOS PDF by Howard Oakley.)
To remove encryption:
- Drop onto Chrome. Enter password. Chrome will render them.
- In Chrome select Print.
- Change print destination to PDF.
The saved PDF is not encrypted and will render in Preview.