What is the FDF file format?
.fdf is a text-based file that contains information filled in an interactive PDF form. An .fdf file is generated when a given form is filled in and exported - please note that .fdf files are not created automatically. Form data may be exported by choosing an appropriate option from the Adobe Acrobat or Adobe Acrobat Reader menu.
Every .fdf file begins with the header %FDF- followed by a version number (for example %FDF-1.2), mirroring the %PDF- marker used by PDF documents. Its body uses the same object and dictionary syntax as PDF but contains only a single /FDF dictionary that maps field names to the values a user entered.
PDF files are larger and have a more complex structure compared to .fdf files. .fdf is used only in situations where you need to extract from a document only the information entered by a person filling in a given form. Such data can then be imported back into the original PDF, submitted to a server, or processed as needed.
An XML-based sibling format, .xfdf, stores the same field data in standard XML and is easier to parse with generic tools. Both formats are part of the ISO 32000 PDF standard.
Security & safety
RISK: LOWAn FDF is a small data file of form-field values, not an executable, so it is low-risk to open or inspect. The realistic concerns are privacy and provenance, not malware: an FDF can carry personal data someone typed into a form (names, addresses, IDs), so handle it like any form submission. Like PDF, the format can technically reference a URL or the source PDF - so only import FDF data into forms from senders you trust, and be wary if an unexpected FDF points to an external PDF. The everyday risk is simply confusion: people expect a document and get a tiny data file.
Format details
in a nutshellPrograms that open FDF files
Windows3 apps
Adobe Acrobat
Paid Open the source PDF, then choose Import Data and pick the FDF; also exports/creates FDF. Full forms toolkit. 
Adobe Acrobat Reader
Freemium Open the original blank PDF form, then More > Import Data (or Forms > Manage Form Data > Import) and select the FDF to fill the fields. Free. 
A text editor (Notepad / Notepad++)
Open-source Open the .fdf as text to read the raw field names and values - useful to inspect or migrate data (it's PDF-like text). 
macOS2 apps
Adobe Acrobat Reader
Freemium Open the source PDF form and Import Data > select the FDF to populate the fields. Free on macOS. 
Linux2 apps
pdftk (PDF Toolkit)
Free Command line: 'pdftk form.pdf fill_form data.fdf output filled.pdf' merges the FDF data into the PDF. Also dumps form data to FDF. Technical details
deep spec| File header | `%FDF-` at byte offset 0, followed by a version string (e.g. `%FDF-1.2`) |
| MIME type | `application/vnd.fdf` |
| Developer | Adobe Inc. (Adobe Systems) |
| Syntax model | PDF object syntax - dictionaries, arrays, name objects, and a cross-reference table, identical in structure to PDF |
| Root object | Single `/FDF` dictionary containing a `/Fields` array of field-name/value pairs |
| Typical file size | A few hundred bytes to a few KB - contains only field data, not document layout or graphics |
| Linked PDF reference | The optional `/F` key inside the `/FDF` dictionary stores the filename of the associated source PDF |
| Supported field types | Text, checkbox, radio button, list box, combo box, and digital-signature fields |
| Annotation export | Can carry PDF annotation data (comments, markups) extracted from a document, not only form-field values |
| HTTP submission | Submitted to web servers via HTTP POST with content-type `application/vnd.fdf` |
| XML alternative | `.xfdf` (XML Forms Data Format) encodes the same field data in well-formed XML, with the same ISO 32000 basis |
| Character encoding | ASCII/Latin-1 for structural syntax; PDF string objects inside FDF may use UTF-16 BE for Unicode content |
| Released | 1996 (Adobe Acrobat 3 / PDF 1.2); standardised within ISO 32000 |
| Open standard | Yes · royalty-free |
| Specification | www.iso.org |
FDF conversions
Community Q&A
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