Are you expecting to be able to sign a document by merely importing an FDF? If so, that's not possible.
No i'm not. I'm asking if there's a way (any) to resign the pdf that is filled by the fdf.
If the document wasn't locked when previously signed and you were the one who signed, you should be able to right-click a signature field and clear it, and then sign it.
Thanks for the answer.
Let me explain a bit more about my problem.
A client acess the pdf template by a url. The pdf have a signature field and a button that send the fdf to a server by a http post.
Now that i know that the fdf file dont get the signature field content, i need to know if there's another way to have the signature.
I suspected that the fdf file can hold the signature because in the fdf format specification it says:
2.1 FDF Catalog
The FDF catalog dictionary is used for signed FDF files, which were introduced in Acrobat 6 (PDF 1.5). The signature syntax is described in the PDF Reference.
TABLE 2.1 Additional entries in the FDF catalog dictionary
SEMANTICS (Optional, always indirect; PDF 1.5) A reference to a signature dictionary that contains a signature of the FDF file contents. The syntax for this signature dictionary is defined in the PDF Reference.
You can somehow get a different kind of FDF, which relates to an append save (can't remember the details). This isn't easy to analyse (no easier than a PDF if not harder) but it contains all the info added to the PDF as a part of signing it.
So, if you append this stuff to the exact same original file you get it signed again. There is absolutely no other way to sign, other than to go through signing processing - because otherwise anyone could fake sign just by doing whatever you could do. Don't even know if this still works, haven't heard about it in years. Mostly if people sign they submit the whole PDF and done.
I see now. It is possible to set up a submit form action to include the incremental updates, which can then be extracted from the FDF and concatenated with the original PDF. The FDF Toolkit has a method for extracting the appended saves, but this has not been updated for many years.
The advantage of this method over simply submitting the entire PDF is less data needing to be send from client to server and more ready access to the form data, but the former may not be so important these days and you can still get access to the form data server-side by reading the PDF.