This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
Install the extension.
To enable the extension you have to set plugin.tx_powermailpdf.settings.enablePowermailPdf = 1. You can add this to the TS of a page where the powermail formular is located. This is important if you have more powermail forms but just one needs the pdf filling feature.