For Linux and macOS computers that you are allowed to install software on I recommend the pdftk command line tool.
Ubuntu family:
sudo apt install pdftk
brew install pdftk-java
If you're on a Mac, the built in Preview tool has had the ability to merge and manipulate PDF documents for years.
For Linux and macOS computers that you are allowed to install software on I recommend the pdftk command line tool.
Ubuntu family:
macOS with Homebrew: