How to concatenate various jpgs pngs and pdf into single pdf leaving their sizes as they where?

I am trying to make a kind of dossier out of various pngs,jpgs and single page pdf images. It has to be a single pdf file, but it doesn't matter that results into something conveniently printable. As such, each page can be whatever size and format whatsoever. I have seen answers here that show how to make them into nice same-size pages, or center them, or such, but my interest is exactly the opposite. The order is already solved as the filenames are nicely numbered. If I try with pdfunite (after a convert), it makes them fit into letter paper pages. Using convert directly also makes them fit them into letter paper pages, but it crops images that are to wide.