This nasty virus has been around a while now, I was recently sent an update and reminder it is still out there - so be warned, be ready and be aware. As you will see from the linked article, most people are caught because they think they are opening a pdf when in fact they are executing a program. Mostly this is because Microsoft foolishly set the defaults to hide file type. If you want to fix this in Win7, open Windows Explorer, click on Organise, Folder Search Options, View and take the button out of the box for hide extensions for known fle types, for Windows 8, 8.1 &10 simply click on View and put a tick in the box next to File name extensions (in you are still using windows XP, click on Tools, Folder Options, View and take the tick out of hide extensions for known fle types).
It is recommended that you always save any attachment you receive, then when you open your documents (or wherever it is you saved the attachment) you will immediately see if it is what it purports to be, eg invoice.pdf is pretty safe, but invoice.pdf.exe is absolutely dangerous).
If saving the attachment is too difficult for you, then at least "hover" your mouse pointer over the attachment and you should see the real name.
Don't get caught. Do a backup of doc, photos, music, etc that you can't afford to loose. Save attachments to your hard disk and then browse to them, if you right click on the file you should be able to scan it with your anti-virus software.