Hi Kirti Rastogi,
Because this question is specifically about the Open XML SDK library, questions should be directed to the repo for this library and not here on Q&A. Here is the issue list: https://github.com/dotnet/Open-XML-SDK/issues
I can tell you that the Open XML SDK library does not do any conversion of PDF or WordprocessingML to Rich Text. While it can consume and allow manipulation of WordprocessingML (.docx packages) it does not contain Rich Text capabilities. You might find another library or tool to help with that but the Open XML SDK won't be able to do it.
You might also post your question to https://stackoverflow.com/questions/tagged/openxml-sdk or just Stack Overflow in general and ask if anyone knows of a programmatic tool for Rich Edit conversion.
Best regards,
Tom Jebo
Microsoft Open Specifications Support