Sometimes you just want to create a PDF from a couple of scanned document papers. One solution could be of course to embed these pages into LibreOffice / Word and then use a PDF printer to get it. Some people though like tools on the shell.
Therefore we can use imagemagick, which is probably already installed on your system.
It provides a command line util called
convert first-image.png second-image.png image.png result.pdf
Maybe a bit too easy.