How to change default PDF viewer on Nook Color?
Find the app that took over the PDF intent in Menu -&gt; Settings -&gt; Applications -&gt; Manage applications -&gt; All, and then tap on Clear defaults button.  Next time you click on a PDF file, it should prompt you which app to use.  Check the Use by default for this action box, and tap on ezPDF Reader.

Alternatively, you can try uninstalling and re-installing the ezPDF Reader app.  It should re-register its own intent for handling PDFs, and prompt you for an app next time you try to view one.