I use Yahoo Mail on an XP Platform protected by Avast Free AV and Comodo Personal Firewall. For some strange reason email attachments - even just pdf's from one or more trusted sources - are now being prevented from downloading onto my hard drive (after automatic scanning) as a prelude to being readable. The Avast Forum people have so far been unable to think of an explanation that I have not tried so I am desperately looking around for one or more alternatives.
The only thing that has changed recently is that deep scans by Ad-Aware and Avast uncovered some infected files which they have quarantined. I have upgraded to the latest Avast version and have temporarily disabled the Avast Online Security extension showing on my Chrome browser list. All to no avail!