Nice exercise, and as a local solution obviously better than all this "give your data and trust me" stuff. But why must everything nowadays be a browser-based tool? I'm still happy with tools like pdfunite from https://poppler.freedesktop.org/
totally understand. I think distribution on the Web is a lot easier than building desktop applications for Linux/Mac/Windows.
Also, sometimes I need to merge pdfs when I'm on my Android device, which would be another target to build an application for. Web makes it easy for me across a huge variety of devices