flpsed is a WYSIWYG PostScript annotator.  flpsed can add arbitrary text lines
to existing PostScript documents, but cannot remove or modify existing elements
of such documents. The added lines can later be re-edited with flpsed, either
interactively or via batch-processing (the latter is useful for repeatedly
filling in forms). Text lines can also be imported from other documents that
have been modified with flpsed.

Using pdftops, which is part of xpdf, one can convert PDF documents to
PostScript and then add text to them using flpsed.  flpsed is thus useful for
filling in forms, adding notes, etc.