i have a look at the vps history and i notice that the number of :pdf and swf files that carry viruses are increased,so why avast on access scanner dont scan swf and pdf by default and this thing must be repaired in the next avast 5 8)
Who did say that to you? 
Mine shows PDFs are scanned in the default extension set, see image, scroll further down and you will see .swf are also scanned.
So I guess it would depend on your settings, see mine in the image also, clicking the Show… text displays the default set.