First of all make sure you have imagemagick installed, otherwise checking won’t hurt:
$ sudo apt-get install imagemagick

Then in this case, I would like to join multiple image files of scanned documents with sortable file names (e.g. file01.jpg, file02.jpg, etc.). Finally to join those files together in A4 PDF format we do this in terminal:
$ convert -page A4+0+0 file* OutputFile.pdf