BEWARE - seriously - do not, ever, open any attachments in an email unless you are CERTAIN that the email is legitimate.
There are hundreds of fake, spam, emails, that may look genuine; but once you open the attachment then your computer is infected with a virus.
Some of these virus's lock your computer, so that you cannot access your data unless you pay a ransom (typically tens or even hundred of pounds) to the criminals behind this. This is called 'ransomware'.
If you get any emails of this type, then send it to Junk and do not open the attachment.
Many of these spam emails say things like "Invoice for payment", "Your order", etc.
Some of them start off with "Re: " in the subject line so that they look like they are replying to your email.
Also see the useful emails from Chaa006