In the app's Library find the document that you would like to edit and open it. In the bottom right corner of the screen press the "add page" button. You will be transferred to the scanner screen of the app. The previews of all previously saved pages of the document will be shown below.
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 Trash can button in the bottom left corner - the page will be deleted.
Pages in a document can be rearranged at any time. In order to change a page's position please tap on it, open the Edit screen (left top corner) and tap a Reorder pages button.
Tap a page and move it where necessary. 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 "Done" symbol in the top right corner of the screen.