If you want to add a blank page to the document, you'll need to scroll down to the bottom of the document and find the circular "Plus" icon circled below:

Once clicked, you'll see an Add New Pages menu pop up. 

Selecting the first dropdown option allows you to choose which Page Type you'd like to add. Choices range from blank pages to musical staves, lined pages to Venn diagrams!

The second dropdown option allows you to adjust the Page Size.
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The third option allows you to choose between Landscape or Portrait Orientation.

You can also select how many pages you want to add of your chosen type with the Pages option.

Lastly, you have the option to choose the Background Color of the page you're adding:

You can choose colors with our swatches option or create your own mix with the Color picker.

Once you've added those pages, they'll appear at the bottom of your document.

If you need to re-order or remove pages from the document, you can do so by loading the document into the Split & Merge tool from the dropdown menu in the menu bar.
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For more information on Split & Merge, check out the help article available here.