… because there are oh-so-many tablet Macs. There are ModBooks and there are non-Apple tablet machines running Mac OS X. So, if you have such a device and are looking for something that provides a surface on which to draw, write, etc., with various “pens” and with the ability to save to PDF and PNG, MacWJ may be of use to you. It’s open-source under MIT license.
MacWJ requires OS X 10.5 or newer and some sort of tablet-pen input, whether a tablet-form computer or a Wacom graphics tablet or something else of that sort. Ready-to-run recent build(s) are available on bitbucket, as well as the full source code.