Whilst I'm not a Mac user, the concept that a file can't be scanned (given the reason) I believe is the same.
1 - Not a hard and fast NO
2 - No
3 - Since 1 & 2 above are No - I don't believe it is likely these would be targeted, be that for a Mac or Windows system.
Whilst this is a very old windows related topic - See
http://forum.avast.com/index.php?topic=35347.msg297170#msg297170 this topic for more information on why files can't be scanned.
Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.Many programs (usually security based ones) password protect their files for legitimate reasons, there are others (and avast doesn't know the password or have any way of using it even if it did know it).
By examining 1) the file name, 2) reason given by avast! for not being able to scan the files, 3) the location of the files, you can get an idea of what program they relate to. You may need to expand the column headings to see all the text.
- Decompression Bomb, a file that is highly compressed, which could be very large when decompressed. This used to be a tactic long ago to swamp the system.
The name really is the most dangerous thing about this and I wish they would change it or simply not report it, a real PITA.
These highly compressed files are generally 'archive' files which are inert, don't present an immediate risk until they are unpacked. If you happen to select 'All packers' in your on-demand scans then you are more likely to come across this type of thing. Personally it is a waste of time scanning 'all packers' and that is why it isn't enabled by default (may differ in the Mac version of avast).
- Corrupted Archive file, this could simply mean that avast is unable to unpack it to scan the contents of the archive and assuming it is because it is corrupt. Even if it were corrupt there is nothing that a user can do to resolve any corruption, short of replacing the file..