Multipage .pdf file creation

I have 24 .png files that I want to wrap into a single 24-page .pdf file without losing image resolution (as I would if I pasted the .png files into 24 pages of a Word document and then .pdf printed on my Mac). Any way I can do this for free and legally? I currently have access to both PC, Mac, and linux on a very slow computer; I have a slight preference for Mac.