It depends on the type of PDF. If it's an XFA PDF, i.e. you can open the PDF in LiveCycle Designer, then you could simply save it as an interactive PDF. Since this is the PDF Generator forum maybe you converted a document from a different format (e.g. MS Word) into a PDF? If that is the case then this is more complicated. You'd have to try to import the PDF into LC Designer and then add the fillable fields you need.