The .fdf file extension refers to a specific document file that is related to Adobe. FDF files are document files that can be opened or created using Adobe Acrobat reader. FDF means "forms data format". It is a forms format specifically for .pdf files. FDF files are used for text files exported from .PDF files.
FDF files are fairly smaller than .pdf files because they mainly contain the forms field data and not the entire form. In other words, .fdf files only have the "contents" of the .pdf without the complete package. It is like a thought in your head without putting it in the paper. Some editorial managers in browsers use files with .fdf extension to ensure that the file opens in Adobe Reader and not within the browser. Only the Reader can open .fdf file, but the users can export it in .pdf.
FDF file format is now popularly used as an attachment in spam messages. If you receive an attachment in your email and the size is fairly small, opening it is not advisable and is considered impractical. Small FDF files are quite harmless, but are also useless. However, .fdf files that consume large memory size should be scanned first before prompting it to your Adobe Reader. You may resume downloading the .fdf file once you're done with the necessary security check. Converting the FDF file to PDF is also recommended to make the file more accessible.
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If you have additional information about the FDF file format or software that uses files with the FDF suffix, please do get in touch - we would love hearing from you.