You can easily merge two or more PDF's to one on Linux CLI - private and secure, without having the risk that any suspicious webservices crawls your data.

1st. option: Therefore install the package poppler-utils.

sudo apt insall poppler-utils

The use pdfunite:

pdfunite <inputfile-1> <inputfile-2> [<inputfile-x>] <final-file>

2nd. option: Otherwise you can use pdftk:

sudo apt install pdftk

With this command structure:

pdftk <inputfile-1> <inputfile-2> cat output <final-file>

Paritally sourced from linuxopsys.com - Merge PDF