Option 1: Copying and pasting pages
1. Open the source document and tap the four-square icon in the top-left corner to enter thumbnail view.
2. Tap Select in the top-right corner, then select the pages you want to copy/move. You can quickly select/deselect a row of pages by swiping across it. Tap Select All to select all the pages, for example when you want to merge the entire document into another.
3. Tap Copy. If you want to move those pages instead, also tap Trash to delete the originals right away.
4. Tap Done > Close.
5. Open the destination document and go to the page before/after which you want to insert the copied pages.
6. Click the Add Page icon near the top-right corner, choose Before / After / Last Page if you respectively want to insert the copied pages before the current page, after the current page, or as the last page.
7. Tap Paste Page(s).
Option 2: Dragging and dropping
Using Split View, you can drag and drop pages from one document to another.
1. Open the source document.
2. Open the destination document.
3. Tap the small split screen icon on either document's tab, and choose Open in New Window.
4. In each window, tap the four-square icon in the top-left corner to enter thumbnail view.
5. To copy a page: drag its thumbnail to the other window. A green plus icon will appear on the top-right corner of the thumbnail you're holding, signifying a copy action. Continue dragging the thumbnail to a new location in the destination document and release your finger. To copy multiple pages: while holding a page thumbnail with one finger, tap any other page thumbnail with another finger to include it for copying, then drop the group of pages in the destination document normally. Sounds complicated but works very well once you're familiar with it :-)
6. Delete the original pages in the source document if you want to move them instead.