Post your question on StackOverflow instead. Provide more information if you want a decent answer.
Your question contradicts itself.
You say:  "I am able to fill the form" which leads us to assume that there are AcroForm fields in the PDF.
You also say:  "acroFields.getFields() coming as null", but if that is true, you can not fill the form.
 is in contradiction with , hence your question is invalid.