FormForge

FormForge turns a flat PDF into a fillable one without leaving the browser. Load a document and it looks for the places a reader is meant to write — ruled lines, boxes, dotted underscores — and proposes fields there. From that starting point you can move, resize, rename, retype or delete anything, add fields the detector missed, and mark which ones are required.

The export is an ordinary AcroForm PDF: it opens and fills in Acrobat, Preview, Chrome and the rest, with no dependency on FormForge afterwards.