This is one of the oldest entries in my offline notes. I used it quite a bit years ago. Simply said, take several .pdf files and combine them into one file.
As always boys and girls – “man gs” in the terminal before you do this.*
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=final.pdf file1.pdf file2.pdf file3.pdf
*As a side note: some of the options in the above command are not in the man page on my Debian 9 system. Man says you may need to install ghostscript-doc and view the pages in /usr/share/doc/ghostscript/
**Looks like “-o” is a shorthand to replace the “-dBATCH -dNOPAUSE” combination. https://www.ghostscript.com/doc/9.20/Use.htm