I don't have experience with doing this using iWork, but there's third party software for this. Serial Mail is a cheap solution that lets you actually compose the message template in Mail. It should work OK for a few hundred messages if the attachments are reasonably small. Professional bulk mail sending software applications usually do not depend on Mail. I've been using Mailings for quite some time. It does its job reasonably well, but it crashes quite often. The developer was always being helpful, but never actually resolved the issues. I still feel that it's more reliable for sending thousands of messages then pushing them through Apple Mail. Other popular bulk mail software products are: MaxBulk Mailer and Direct Mail, I haven't tried either of these, they cost more and might be more reliable, but if you visit MacUpdate you'll see that both have mixed reviews at best.
All of these should allow you to customize messages including attachments (Serial Mail and Malings certainly allow it). Another option would be a web-based service like MailChimp (I believe that you can use MailChimp for free for less than 1000 messages).
An important caveat: If you want unique messages, each of them needs to be sent individually, and that may take quite some time if the attachments are large. So whatever method you choose, make sure that those PDFs are optimized.