PDF Viewer supports merging two or more PDF files into a new PDF file.
How to Merge Documents in the Document Browser:
Tap on the "Select" button (top-right corner) in the Document Browser.
Select two or more documents that you wish to merge.
Tap on the "Merge" button (top-right corner).
In the new dialog, drag the documents in the the order they should be merged.
Tap on "Done"; a new merged document will be created.
How to Merge Documents in the Document Editor:
Open the document editor by tapping the grid button at the top right
Make sure you are in the edit mode. To switch to the edit mode tap the edit button next to the grid button.
Tap on the plus button at top left.
Tap on the "Select Source Document" button to add the pages from another document.
Save by tapping the checkmark button at top right and choose a save option.
Note: Any changes that are not confirmed will be lost when leaving the document editor!
Document merging is a Pro feature and requires a valid PDF Viewer Pro subscription.
Currently unsupported for merging:
Password-protected files
Image documents
See also: Merge PDF Documents on Android