Sometimes I need to merge multiple pdf files into a single pdf document and there is a handy command line tool called pdfunite that I use to do so. You can install it with:
sudo apt install poppler-utils
Once that is installed you will have access to the pdfunite command:
pdfunite file1.pdf file2.pdf destination.pdf
If you want to combine more than two files you can do that too by just listing all of the files and making sure the last one is the name of the output file.
pdfunite file1.pdf file2.pdf file3.pdf file4.pdf destination.pdf