Open index.html
and include PouchDB in the app by adding a script tag:
<script src="//cdn.jsdelivr.net/npm/pouchdb@7.1.1/dist/pouchdb.min.js"></script>
<script src="js/base.js"></script>
<script src="js/app.js"></script>
PouchDB is now installed in your app and ready to use! (In production, you should use a local copy of the script.)