In the app's Library find the document that you would like to edit and open it. In the upper left corner of the screen press the "+" symbol. You will be transferred to the scanner screen of the app. The previews of all previously saved pages of the document will be shown above.
You can now add pages by scanning them. Please see this guide on How to scan a document.
Every newly scanned page will be added at the end of the page preview list at the top of the screen. Swipe previews of the pages to the left in order to see the new pages.
In order to remove a page please tap the preview of the page and pull it down - the page will be deleted.
Pages in a document can be rearranged any time. In order to change a page's position please tap on its preview and pull it to the right or left. Release when the necessary position is reached.
Note! After you make all necessary changes please don't forget to save the document by pressing the "check mark" symbol in the upper right corner of the screen.