Combining multiple PDF documents into a single PDF document used to be a massive pain. Your options were to either buy Adobe Acrobat, or download some sketchy program that only kind of worked.
So, out of spite, I made this simple program that takes multiple PDFs, and combines them into one, big PDF. That's it. That's all it does.
If just want to download it, executable (.exe & .jar) versions can be found at https://www.nicktriance.com/downloads.