My view is that the spammers must have some motive. If there was an obvious commercial motive from the text in the attachment, such as inviting you to visit some web site, or buy something, then there may be no need to worry. No reason to assume it is any more malicious than that.
I was really just making the point that virus scanners are only really good at detecting viruses once they have been documented. A brand new virus can circulate for a few days before the AV updates catch up. There's not a great deal you can do, other than worry, and worrying is never productive. If it were me then I'd be tempted to run a few different scanners, just for comparison, otherwise just put the back of your mind but be wary of any odd behaviour.
I don't think .pdf attachments are intrinsically any safer than word attachments. My own golden rule is simply to never open an attachment of any type, unless it is something I am reasonably expecting to receive, no matter who appears to have sent it, and no matter who else is receiving it.