You need to read the Acrobat Help entries on creating bookmarks.
You'll use the Bookmark panel (you don't say which version of Acrobat you're using so I can be more specific). You'll choose New Bookmark from the panel menu. You'll right click on the bookmark to change its Properties. You'll edit the Action of the bookmark to Open a web link.
Thanks for the reply,
I'm using Acroabt 9 Pro. I was able to edit the action of the bookmark as you described and it works as expected but I wondered whether it was possible have the link open in a new browser window or tab when clicked? Is this able to be done?
Of course, you can change the URL to whatever you want.
That's great - thanks for the advice