iAnnotate is the 'Rolls Royce' solution and the developer has a free comprehensive user manual that you can download, so you can ascertain whether it meets your needs.
There seems to be a misconception on what "filling" fields on a PDF means. If you are referring to filling fields on a form that has clickable dedicated fields built into the form, PDF Expert is the ONLY app that does this. I have tried them all, someone please correct me if I am wrong.