A general principle of sending email (or any communication) is "The more recipients there are, the more time and care you should take in preparing it." It is ineffective and rude to waste the time of many people, asking them to decipher unclear language or bad attachment formats.
There are many reasons to avoid sending or reading many kinds of attachments sent via email. Lets start with the disadvantages of one particular type of attachment: a Microsoft Word document. It is usually preferable to send plain text in the body of the message or HTML rather than a Word file. See below for how to do that.
There are some "free" programs which allow people using some operating systems under many circumstances with many modern computers to read many Word documents. But some other people cannot use these programs even if they wanted to, and it is the clear goal and practice of Microsoft to continue to modify Word and its other programs so that all attempt to be compatible with them will fail. Remember - Microsoft has been convicted in court of just this sort of monopolistic behavior, and it was sustained upon appeal.
If you prefer typing your documents in using Word, you can "Save as.... HTML" or "Save as... Text" and email the resulting file, rather than sending the original Word file (i.e. one with a ".doc" file name extension). Here are some more detailed instructions: open the document, click on File, then "Save As", and in the "Save As Type" strip box at the bottom of the box, choose "HTML Document" or "Web Page". Then choose Save. You can then attach the new HTML document instead of your Word document. Note that versions of Word change in inconsistent ways -- if you see slightly different menu item names, please try them. To convert to plain text is almost the same -- instead of HTML Document, choose "Text Only" or "Text Document" as the "Save As Type".
Another alternative is to use "copy-and-paste". First "Edit/Select All" in Word and then paste it into your email program.
Often it is better to put a file on a web site and then send a pointer to it (a URL) rather than attaching it to an email message. The email is smaller and faster, and when the recipient gets it and clicks on it they will always see the latest version.
When sending spreadsheets just to be viewed, it is best to save them as PDF or HTML files rather than, e.g. Excel), unless you know all the recipients prefer getting them in a native format. If you expect them to edit the files and send them back, you need to find out what software they have available and what they prefer.
Sending pictures is generally best done as standard .jpg or .png or .gif image email attachments, or as PDF files if they need to be printed (until standards like SVG are widely implemented). Sending wmf (windows metafile) images causes problems for recipients who don't have the proprietary software necessary to handle them.