How to send an electronic notice
The correct (or at least polite) way to send an electronic notice depends at least partially on the medium being used. You can get away with more on a web-based system than you can in an email link. You probably shouldn't, however.
There are two main rules you should follow:
People don't want to have to open a separate application to read your notice and often the required application isn't even installed on their computer. Another reason to send plain text is file size. A Microsoft Word document is typically several thousand times bigger than the equivalent plain text. Also, you almost never need anything more than plain text to make your point. If, as is all too common, you send a Microsoft Word attachment you may also be spreading viruses.
A common misconception among computer users is that everybody use the same applications they do. The canonical example of this is Microsoft Word. Many computers these days come with Word preinstalled. This does not mean, however, that all computers have Word. Many people use a free equivalent, such as OpenOffice or perhaps something a little more esoteric, such as LaTeX. Asking people to install particular software to read what you send (which is basically what you are doing when you send them an attachment) is exceedingly impolite, especially when you consider that most such software is rather expensive and there is generally a reason the recipient uses something different.
Word processor files tend to be significantly larger than the plain text equivalents. This is because of all the default formatting information, possibly embedded fonts, metadata (author name, etc.) and so on. An empty Microsoft Word document is often larger in than an entire novel written in plain text. This isn't so much of a problem if you're sending an email to a single person, but if it's going out to a large list of recipients (as notices tend to do) it wastes a huge amount of bandwidth and mailserver disk space.
Yet another reason to avoid sending notices as attachments is viruses. Many applications (such as Microsoft Word) allow embedding of executable code in their files. This gives viruses a way to spread themselves. Avoiding such attachments is a good idea unless you know them to be virus-free.
If you cannot avoid including an attachment (a map to the venue of a talk, for example) it may be a good idea to put a link to a website in the notice rather than the attachment itself. If an attachment is absolutely necessary, it should accompany a plain text message rather than being the entirety of the notice. At the very least, give a brief description of what's in the attachment and why it isn't in the plain text.
Plain text also means minimal formatting (on the web) or no formatting at all (in an email). People reading notices would rather not look at seventeen fonts in forty three different colours and sizes. It makes it difficult to read.
This rule, unlike the previous one, allows no exceptions. If you don't have the courtesy to tell people what the notice is about, why should they bother to read it? A subject line is there to tell people what's in the rest of the message. "An important notice" is not a descriptive subject line. "An important notice regarding funding for next year" tells the reader what will appear in the notice and, more importantly, allow him to decide whether the notice applies to him or not.
Have some respect for your readers. A notice consisting of the subject line "See attached important notice" and a 500kb Word document full of different fonts and colours is likely to really annoy a large number of readers, whether it's advertising a meeting about next year's funding or an appeal for people to look for your lost pen.