PDF is fine. To add a PDF natively in iWeb...
• Select the text or object you want to turn into a hyperlink.
• If the Link Inspector isn’t open, click Inspector in the toolbar (or choose View > Show Inspector), and then click the Link Inspector button.
• In the Link Inspector, select “Enable as a hyperlink,” and then choose this option from the Link To pop-up menu:
• A File: Downloads a file to the visitor’s computer. (The file is part of your website; visitors won’t have access to your computer.)
Note: the hyperlink will only be testable from a browser once you've published your site.
An alternative approach is to use Scribd:
http://www.scribd.com/tools/embed_simple
And the free
Posterous blogging platform can also be used to automatically post your PDF to Scribd and embed it into your blog — here's an example:
http://dont-panic.posterous.com
http://posterous.com/faq