There are a number of ways which you can use to merge PDF files. It depends on the Operating System you are working on. On Windows you can use OpenOffice, LibreOffice, PDF Split Or Merge, and many more. All the software are completely free. If on MAC systems, you can go for Automator, Preview, LibreOffice, and many more.
Hi, Usually I use Movavi pdf editor to merge pdf files. It has a simple interface and a wide range of options that allows to use this program even at the office. I have forgotten about any problems with pdf editing with Movavi. Simply give it a try. Hope it will help