Merging PDFs in Node

What do you expect to get when you do Buffer.concat([pdf1, pdf2])? Just by concatenating two PDFs files you won’t get one containing all pages. PDF is a complex format (basically one for vector graphics). If you just added two JPEG files you wouldn’t expect to get a big image containing both pictures, would you?

You’ll need to use an external library. https://github.com/wubzz/pdf-merge might work for instance.

HummusJS is another PDF manipulation library, but without a dependency on PDFtk. See this answer for an example of combining PDFs in Buffers.