To add a new page to a PebblePad portfolio, you can click the tab with a plus icon to the right of your existing pages. When you click the plus icon, you are also given the option to add an existing asset. 

This option allows you to choose an existing PebblePad page or portfolio of pages that you want to reuse.

However, if you try to add a file (such as Word document, a PDF or an image) using this method, unfortunately you will get stuck at a loading message that doesn't disappear.

Instead, to add a file to a portfolio, please add it to your asset store and then create a link from an existing page in your portfolio.