Hello,
Yes, it can scan the files .rar .cab .pdf .xls and .doc format files. However, password protected files would not be scanned.
Once the password is inserted and the file gets executed, the files would be scanned by Symantec.
The Decomposer engines built into SEP are able to interact with the great majority of compression fromats that are in use today, but ther is no way they would be able to interact with them all. Almost every SEP client has entries like yours in its logs.
Also: even if there is a threat inside a zip/container that SEP could not scan, SEP's Auto-Protect would catch it as soon as the malicious file was unzipped.
Final recommendation: it would be a good idea to perform a "disk cleanup" to flush out that recycle bin and other temp locations.
Security Response's official Best Practices:
http://www.symantec.com/business/theme.jsp?themeid=stopping_malware&depthpath=0
Secondly, check this Article:
"Could not scan [#] files inside [path][filename] due to extraction errors encountered by the Decomposer Engines" during a scan
http://www.symantec.com/docs/TECH99755
Hope that helps!!