Creating links from one PDF in a PDF Portfolio to another PDF in the same PDF Portfolio is really pretty simple but the “Preview” window is not the place to do it. The Preview Window does give you access to a few of the editing tools in Acrobat such as the commenting and link tools but, in my experience so far, saving your changes has proven to be unreliable. Don’t worry, there’s a work-around and I’ve already filed a bug report.
You’re much better off clicking the “Open” button to bring the file into the full Acrobat User Interface and creating your interactivity there.
Now you’ll be able to insert a link. In the link dialog, be sure “Go to a page view” is selected even if you are opening another file in the PDF Portfolio.
You’ll see the dialog below.
Your PDF Portfolio should still be open in a separate window, switch to it. Using the Portfolio navigation tools, go to the document you want to link to and click “Set Link”. You should be back where you started.
Be sure to save the file.