You are using a old version of avast. 2014.9.0.2007 is the latest one.
I suggest you use avastclear to remove the current installation and install the newest version

Some file can take a long time to scan.
A pdf that is 25Mb, is not really something you see daily.
Are you sure this is a legitimate file?

The second picture says nothing since it is not a log showing what files could not be scanned.
e.g. if you have a password protected compressed file, avast doesn’t know the password and therefor it can not scan the file.