PDFs can have form fields by using the interactive form features provided by the PDF file format. Form fields supported by PDF 1.4 or higher... PDF/A has nothing to do with that.
Form fill for net framework v4.x using the free iTextSharp v5.x (ref) - sample
Form fill for net v5/6/Standard using the free Kevsoft.PDFtk
or PDFsharp (sample)
or e-iceblue.FreeSpire.PDF
Form fill for net framework v4.x using the free iTextSharp v5.x (ref) - sample
Form fill for net v5/6/Standard using the free Kevsoft.PDFtk
or PDFsharp (sample)
or e-iceblue.FreeSpire.PDF