"Files that cannot be scanned are just that. Files that for one reason or another can not be scanned."
It doesn't mean that they are dangerous or suspicious or harmful. It simply means that these files need another element
before they can actually be opened and examined by your antivirus program.
In your case, the files are password protected and avast! doesn't know the password.
Once the password is supplied, these files will automatically be scanned by avast! before they are allowed to be opened.
There are many reasons for avast! or any other program not to be able to access a file.
In all cases, once that needed missing link is supplied, the file will be scanned before you're allowed to access it and,
at that time, you will either be able to access the file or, it will be blocked if it turns out to be something that will harm your computer.