Earlier quoted context omitted.
I wish this got talked about more, Preview is both crazy good and free. You can split PDFs apart, merge multiple different ones into one file, use password protected files, edit and annotate them, add your signature, and so much more. Best of all, it just works, all the time.
What is the trick to merging multiple PDFs into one? That never seems to work for me.
Probably also accessible via Preview’s Automator/Shortcuts actions if it’s something you need to do frequently.