Hi Kiran,
Unfortunately, there is no way to export/import annotations that will keep the fill opacity different from the border opacity that will persist across different PDF viewers. You can however have an annotation with no fill, that still has a border. If you are looking to set the fill opacity just on your own webviewer instance you can however customize the draw function for the annotations you want to change the fill opacity for. But again these changes won’t be applied in other viewers.
Hopefully, that helps.
Dustin