There are a lot of ways to merge PDF files, even if you don’t have some advanced tools such as a full version of the Acrobat Adobe tool. Local PDF Tools is one of those ways that works like a web app on any web browser.
The difference is that it uses Webassembly to work on your PDF files right inside your browser so your precious data will never leave your computer.
For now, it supports merging PDFs, optimizing PDFs by getting rid of redundant page resources like embedded fonts and images, and extracting information like images from PDFs. All of them can work on multiple files out of the box.