It is very nice to be able to compute this in our browsers without limits or registration. The results are not always perfect but already allows having good time testing remixes or doing karaoke with friends! Thanks for sharing your work.
Depending on your needs, Scribus might help with it's scripting API[1], see ScribusGenerator[2] for example.
Scribus and inkscape can import pdfs but it would be preferable to use a clean "source" format, pdf is AFAIK meant for output, like lossy compressed images.
The feeling is understandable and it could be worrying, but since some years Nicolas seems to be focusing on Shiro games and game dev tooling (heaps, hide). The compiler is developed by the Haxe Foundation and the ecosystem by the community which is not that big but has quite a few talents.
Heaps has it's own API, but other Haxe frameworks[1][2] reimplement the flash API. Some tools[3][4] help to convert AS3 source code to Haxe, and the typing and compiler are helpful to identify issues. So depending on the size and dependencies, conversion can be easy once you get past the main language differences.