Typewriter is originally specified by the PDF reference as special FreeText annotation, where Intent=FreeTextTypewriter. It features opaque letters on transparent background, so that users can fill non interactive forms. Herewith typewriter is implemented natively for PDF, and there's also an Okular specific implementation for other document types. The added tool reuses the inline note UI.
This work was done during GSoC 2018. See https://community.kde.org/GSoC/2018/StatusReports/DileepSankhla for details.