@DavidR and @craigb, the reason I ask if it was working correctly for you (craigb) with 4.8 or v6.0 is because the OP already answered before:

And Asyn is correct; v4.8 is still receiving daily database updates, but they are different than for v5/6, so the interactions / consequences might (are) different.

Avast Team MUST check the database update and resolve this strange issue. “Only” one user reported this, but many more users might have the same problem. Avast is “lucky” this is being reported so quickly.

Still, there is no answer from the OP about what exactly is triggering the problem: whether the specific program to view PDFs, or any PDF viewer would trigger the same issue with this database.