Convert a PNG file to PDF in Ubuntu 13.10, 13.04, 12.04 and Linux Mint

This small command should do the trick.

convert xyz.png xyz.pdf

The convert program is a member of the imagemagick suite of tools. Use it to convert between image formats as well as resize an image, blur, crop, despeckle, dither, draw on, flip, join, re-sample, and much more.

If you dont have imagemagick package installed already, install it by typing the below command in terminal.

sudo apt-get install imagemagick

convert also supports the wildcard format:

convert *.png file.pdf

This will convert all PNG file to a single PDF file.