Open Merge PDFs from the Home screen (or use its own page link).
Tap Choose files and select all the files you need (you can select several at once).
Use the ↑ and ↓ arrows to put the files in the order you want. The first file becomes the first pages.
Optional: tick Add page numbers to number every page of the merged file.
Tap the big purple Start button and wait a few seconds.
When you see Done!, tap Download. The new file is saved on your phone or computer.