This documentation is not using the current rendering mechanism and will be deleted by December 31st, 2020. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
Can only fill checkboxes in pdf file when pdftk is installed
If your template PDF is not compatible with this script, you can process it with pdftk this way:
pdftk modele.pdf output modele2.pdf
Then try again with modele2.pdf.
The pdf links generated in mail may not work properly. This feature is still buggy.