QPDF works well for this. Download and install
qpdf-11.3.0-msvc64.exe from
here, or other versions as needed, or "
apt install qpdf
" on WSL 2 Ubuntu or other Debian-ish Linux things.
The PDF code inside the doc from the following is unaltered after joining, and it's very very fast:
"\Program Files\qpdf 11.3.0\bin\qpdf.exe" --empty --pages map_front.pdf map_back.pdf -- map_combined.pdf
It's amazing what some products waste their time on when "doing things" with PDFs, especially concatenations like this. It's all text in there!