Lossless convert PNG image stack to PDF

Converting an image to a PDF file with ImageMagick can be as simple as:

convert in.png out.pdf

For single images -page letter can leave big margins, which may not be desirable.

Convert image stack to PDF: one PNG image per page:

convert -page letter -adjoin *.png joined.pdf
-page letter
does not upsize images, so tiny images will be tiny on the page, one image per page.