PDF Viewer supports merging two or more PDF files into a new PDF file.
How to Merge Documents
Select a file in the PDF Viewer file browser by long pressing on the cover image of the PDF. This will start the multi-file selection mode.
Select additional files for merging by tapping them. If you want to deselect a previously selected file, simply tap it again to remove it from the selection.
Once you're done selecting files, press the merge icon in the multi-file selection bar at the bottom.
Document merging is a Pro feature and requires a valid PDF Viewer Pro subscription.
Tips and Tricks
The order in which files are merged (i.e. the order of pages in the newly created file) depends upon the order in which files are selected in the browser.
The destination folder in which the merged file is created is the folder that is open at the time of pressing the merge icon.
Selected files can be in the same directory, but you can also select files across multiple directories.
File merging works not only for local files, but also for files on SD cards, USB sticks, Google Drive, and Dropbox.
Currently unsupported for merging:
See also: Merge PDF Documents on iOS