Break up your distribution list into obvious compartments and send separate e-mails to each list. I notice that a lot of publicists have print, electronic and internal contacts, which could be easily separated. I'm repeating much of what he says there, but Dan also has several useful tips for writing effective PR e-mails. You should read it. A decent e-mail program will do this automatically if you check a box in its Preferences panel.
Attachments are fine.
I haven't met the file format yet that my Macintosh can't read. But the problem is, attachments get thrown into another folder , and I have to launch a different program to look at them. Which means I'm less likely to look at the attachment. So do yourself a favor. Just send plain text.