Source code
The full source code of Plain PDF lives at github.com/lazytitan30/plainpdf, under the GNU General Public License, version 3. Anyone who copies the app must keep it just as open, and that is the point.
Plain PDF was made to be passed along. Fix something, translate it, put it on the phone of someone who is tired of ads, and whoever comes after you receives the same freedom you did. Good tools grow the way neighbourhoods do: one person looking after the next.
Questions about the code are welcome at support@plainpdf.app, or as an issue on GitHub.