The .FDF file format is better known as the 'Forms Data Format', a format created by Adobe Systems which is used to export data from PDF documents.
When a user clicks the submit button on a PDF form, the information is extracted from the PDF and converted to the FDF format. This information (which is significantly smaller than the original PDF as it only contains the data) is then sent to a remote server, processed, and returned to the form.
To create a FDF file, select Advanced > Forms > Export Forms Data from the Acrobat menu.
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