flpsed is a WYSIWYG PostScript annotator.
You can’t remove or modify existing elements of a document.
flpsed lets you add arbitrary text lines to existing PostScript documents (PostScript is a registered trademark of Adobe Systems Incorporated).
Added lines can later be reedited with flpsed.
Using pdftops, which is part of xpdf one can convert PDF documents to PostScript and also add text to them.
flpsed is useful for filling in forms, adding notes etc.
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flpsed is released under the GPL.
Actually it is more a PostScript annotating tool than a PDF one.
When you first save an annotated PDF document,
it gets converted to PostScript.
If you want to resume editing such a document,
you and the tool have to deal with the PostScript file.
You lost the nice document structure diagrams,
PDF viewers show you for certain documents.
And: PDF resp. PostScript annotating is not filling out PDF form fields.
This tool lets you write text at any location within such a document,
and it does not help you finding PDF form fields, as PDF viewers do.
I still find this utility very, very useful.