Please make sure that you have subscribed to the Document 365 service and are signed in your account.
1. In the "Explore" tab, tap the "Merge PDFs" option.
2. Select 2 or more files and tap the "Next" button on the bottom of the screen.
3. Then you will enter a page where shows the file order for merging. You can drag the file to rearrange the file order.
4. Then tap the "Merge" button to start merging.
5. The output file will be saved in the "Split & Merge" folder.