Thank you for your post. Unfortunately, Avenza Maps does not officially support any password-protected PDFs which is why you are getting this error. iOS devices are a bit more lenient in opening password-protected PDFs, and it’s possible that the creator of those maps only tested them on an iOS device. In order to use these on your Android device, you will need to reach out to the provider of those maps, and ask them to either provide you with a non-password-protected version, or a GeoTIFF export (if they are concerned about the raw file being altered).
I apologize that I cannot be of more assistance, but please let me know if you have any other questions or problems!