According to a popular Office "expert" it is possible to include an attachment in a mail merge but recipients might have problems opening them, due to security issues. Unfortunately, the expert is wrong. Yes, the recipient will be unable to open the attachment, but not because they fear viruses. Mail merge doesn't support attachments - if you add one, it's sent as an icon in the message body.

Other points to remember during a mail merge:

1. Use HTML formatted messages when sending email merges. This will avoid the annoying email security dialog that requires you to press "Yes" for each message.
2. Some versions of Outlook don't handle URLs correctly in an merge. They'll be blue like URLs but won't be clickable. If this happens to your merges, make sure you have the latest service pack for your version and switch to plain text if it's still a problem.

There are two possible solutions if you need to send an attachment- use a mail merge utility that is designed to send attachments or use VBA.

Mail merge utilities:
http://www.slipstick.com/addins/mail.htm#massmail

How to mail merge attachments with VBA (includes the VBA code needed):
http://word.mvps.org/faqs/mailmerge/MergeWithAttachments.htm