notfoundaj01/pdf-merger ? reverse-engineered prompt

Build me a simple PDF merging tool in Python.

I want to be able to pick a few PDF files, put them together in the order I choose, and save the result as one new PDF. Keep it really straightforward, something I can run from my computer without a lot of setup. If it makes sense, add a small interface or an easy way to choose files and the output name, but it can also be a clean script if that is simpler.

Please make it handle basic errors nicely, like when a file is missing or not actually a PDF, and tell me clearly if something goes wrong. If you need to, look up current docs online for the best way to merge PDFs in Python. I’d like the code to be easy to read and use, with a short README explaining how to run it.