I'm thinking about building an App that needs to display PDF among other things and I'm currently considering Flutter as a platform.
How likely is it that PDFTron will in the future support Flutter directly?
Thank you for the feedback, we take into account information like this to help us plan our road maps.
While at that this time there is no plan to provide a specific DART API, our PDFTron iOS/Android widgets can still be used in Flutter apps.
So PDFNet iOS and Android SDK’s do support Flutter now, but not through a DART API.
FYI: You can now use PDF & Office document viewer in Flutter 
https://www.pdftron.com/blog/flutter/build-a-document-viewer-in-flutter/
https://github.com/PDFTron/pdftron-flutter