If you have some kind of announcement or invitation to send out to people:
1) If it doesn't have complicated formatting or a picture or something, send it as plain text. Requiring people to deal with an attachment solely because you couldn't be bothered to cut-and-paste the content from Word into your email editor is just rude.
2) If you can't send it as plain text? Send it as a PDF. That's far more universal a format than Word, and besides your document will look a lot better than if I have to open it using OpenOffice, which can't cope with your fancy-pants templates and crazy fonts. Making a recipient switch computers to fiddle with your attachment is rude and inconsiderate.
3) If you are attaching a PDF to your email, include some descriptive text indicating what the attachment is generally about in the body of the email. Forcing me to jump through the hoops of dealing with an attachment only to discover that it is completely irrelevant to my life is rude, inconsiderate, and REALLY GODDAMN ANNOYING!
Eventually, I will get fed up and stab you in the face with a bazooka.
Also? Don't send large attachments if you can send a link instead, especially not if you're emailing a list with a lot of members. The recipients won't care, but if you waste enough resources this way the admins will hunt you down and cram your sensitive bits into a very small hard-drive casing, and you will be sad.
Tips for happy correspondence, y'all. Don't make your friends all stabbity. *eye-twitch*