Why are attachments a bad idea?
  1. Device compatibility
    The sender has no control over what device or software the recipient is using and, as of 2013, the majority of domestic users are receiving e-mail on smart phones or tablets which probably do not have the software to view the attachment.

  2. Software compatibility
    There are many versions of whatever software is being used. There is no guarantee that the recipient is using a compatible version. However, the majority (possibly all) devices can read pdf files.

  3. Speed
    Many e-mail clients, especially those running on smart phones or tablets, do not automatically download attachments, doing so requiring an additional step or two and, depending on bandwidth available (not much when out in the sticks using gprs), this can take an appreciable amount of time. In fact it is usually not worth the effort.

  4. Version control
    Having sent a word document as an attachment you get amendments returned then need to send a revised version which is then redistributed. Which is the definitive version? A better solution is to store the definitive version on a shared facility e.g skydrive (for Hotmail users), google drive (for gmail users), dropbox , a web server, etc., then send a link to the file.

  5. Virus detection
    Most reputable Internet Service Providers and e-mail hosting systems filter e-mail for spam and viruses which involves unpacking the e-mail, subjecting it to various filters and spam/virus detection systems, repackaging the e-mail and sending it on its way. HTML e-mail as used by Hotmail, Yahoo, Gmail etc., is in itself an e-mail attachment on top of the plain text e-mail headers, so adding another attachment to the attachment increases the chances of it being mangled and slows the system down.