The merge command will combine input files into a single output file.

$ pdal merge <input> ... <output>
--files [-f] arg  Non-positional argument to specify filenames.  The last
  file listed is taken to be the output file.

This command provides simple merging of files. It provides no facility for filtering, reprojection, etc. The file type of the input files may be different from one another and different from that of the output file.