Form Flattening is a technique developed by Appligent in 2001; whereby, the data contained within a PDF form field is drawn directly to the page as page content; and, the original form field is removed from the document.
PDF form fields; sometimes referred to as AcroForms, are a type of PDF annotation similar to hyperlinks and notes. PDF annotations are located in a logical layer above the drawn content layer of the page. Form Flattening remove the form field annotation; and, draws the appearance of the field data into the page content stream.