Fillable PDFs such as application forms and order forms hold their fields in one of two form models, described below, and a signed PDF holds at least one field even when nothing on its pages looks like a form. In an AcroForm, what you see in a field is a picture of its value: the PDF standard has widget annotations draw the fields, and each widget holds a small drawing, its appearance stream, which “shall be consistent with the object’s current value as a field”.
What is the difference between AcroForm and XFA?
AcroForm is the form model of the PDF standard, which calls an interactive form “a collection of fields for gathering information interactively from the user”: fields listed in the document’s form dictionary, with widgets on the pages. XFA, the XML Forms Architecture, came with PDF 1.5 and stores the form as an XML package inside the file. A dynamic XFA form sets a flag that “specifies whether the document shall be regenerated when the document is first opened”, so the viewer builds the pages you see from the XML.
Where do you see form fields?
Poppler’s pdfinfo prints a Form line. On the demo invoice made for PDF Textfix it reads Form: none. On the demo signed note it reads Form: AcroForm, because a signature is a field too: “The signature dictionary corresponding to each signature shall be the value of the form field”. The note’s catalog and its one field, a signature widget with a zero-size rectangle:
1 0 obj << /Pages 3 0 R /Type /Catalog /AcroForm << /Fields [ 4 0 R ] /SigFlags 3 >> >>
4 0 obj << /Type /Annot /Subtype /Widget /FT /Sig /T (Signature1) /V 18 0 R /F 132 /Rect [ 0 0 0 0 ] /P 5 0 R >>In PDF Textfix, a document with form fields gets one entry in the Issues pane (⌥⌘I): “Form fields can’t be filled in PDF Textfix”, with the detail “Values typed into the form would not be saved. Fill the form in another app, then edit its text here.”
Why can’t you edit text in a PDF form field?
Rewriting what a widget shows would not change its value, and the next viewer to redraw the field would bring the old value back. So PDF Textfix locks that text: “This text belongs to a form field or a comment, which viewers draw again from its own value, so it cannot be edited here.” Fields take no clicks or typing, and Save copies every field as it was. A form that viewers rebuild as they open it stays locked until you choose Allow Editing… in the banner and confirm.
How do you change what a form field shows?
Fill the form in a forms app, then fix the page’s own text in PDF Textfix; the help page lists the common lock reasons. To turn field values into page content, flatten the PDF first. Fillable versus editable PDFs compares the two kinds of text, and a certified PDF can forbid any change except filling its fields.
Questions about form fields
Why is the text I typed into a field not visible?
Often the field holds the value but no appearance stream to draw it. The form’s NeedAppearances flag is “A flag specifying whether to construct appearance streams and appearance dictionaries for all widget annotations in the document”, and a viewer that ignores it shows the box empty. Open the file in a viewer that builds the appearances, such as the app the form was filled in, or flatten the PDF there so the values become part of the page.
Is a signature a form field?
Yes. A digital signature is the value of a signature field, even when its widget draws nothing.
Can I fill in the form while I edit its text?
No. Values typed into a field would not be saved, so fields take no typing.