To merge two or more documents follow these steps:
1) Open one of the PDF documents
2) Now tap the Merge icon (Bottom bar, second from right side. It looks like an arrow with two merging lines)
3) In the opening dialog choose another PDF document
4) Now you can select individual pages to merge or simply tap "Select all" on the bottom-left to merge the complete PDF
5) Finish with tapping "Done" on the top-right and save it as a new file or overwrite the initially opened document