How do I convert a PNG to PDF with only the default packages on 11.10?
convert xyz.png xyz.pdf should do the trick.
See man convert for more options.
If you want to convert multiple images, e.g. png files, into one single pdf use convert with the specified pdf filename at the end
convert *.png mydoc.pdf
It will merge all png files into a sinlge mydoc.pdf file in a descendant order.
In 18.04 LTS, open image using ImageViewer; then print to file as a PDF.