I noticed that many people have reported the problems caused by the pdf attachment. Unfortunately, I am not able to solve it or your question that "We could take PDF's out of the scan list for the OnDemand Scanner, but, we are unsure as if this is a good idea or not?". But I found several posts which on similar issue, you may have a check.
Hope it can offer some useful information.
I think you meant On Access Scanner
Try to install Patch 4 as it has been fixed some performance issues.