A fillable PDF has form fields: boxes that sit on top of the page and take what you type, in any viewer that supports forms. An editable PDF, in everyday use, is one whose own text you can change: the printed words on the page, which only an editor can touch. The two live in different parts of the file. A field is an object laid over the page with a value of its own; page text is drawn by the page’s content stream. That is why Preview can fill a form but not fix a typo in the paragraph above it, and why a field you cannot change is usually read-only, flattened, signed, or part of an XFA form your viewer cannot draw.
What is a form field, and why is it not part of the page?
The PDF standard calls the forms most files use interactive forms: “An interactive form (PDF 1.2)—sometimes referred to as an AcroForm—is a collection of fields for gathering information interactively from the user.” Each field has a value (what you typed) and an appearance (how it is drawn), and it is placed on the page through a widget annotation, a rectangle that floats above the page content.
The two can drift apart: qpdf’s documentation has an option for a file whose “appearances are out of date with the values of the form”, which redraws them. A viewer can redraw a field from its value at any time. The page underneath never changes when you fill the form.
What is the difference between AcroForm and XFA forms?
AcroForm fields are the standard kind described above. XFA is a second form technology: “PDF 1.5 introduces support for interactive forms based on the Adobe XML Forms Architecture (XFA).” An XFA form carries its layout and logic as XML inside the PDF. Dynamic XFA forms are, in Qoppa’s description, “designed to be rendered dynamically from the drawing and layout information present in the XFA definition”, and the standard marks them with a flag, NeedsRendering, that tells a viewer to build the pages when the file opens.
That is why some forms open blank, or on a single page of text, outside the apps that render XFA. The page a dynamic form carries for other viewers is often just a notice that reads, in part: “If this message is not eventually replaced by the proper contents of the document, your PDF viewer may not be able to display this type of document.” The form is still in the file; the viewer cannot draw it. Qoppa’s workaround is to save the dynamic form as a static one in the app that designed it: “The static form will be able to open in PDF Studio as well as most PDF viewers.” XFA is also on its way out: Datalogics notes that “ISO chose not to include it in PDF 2.0 (ISO 32000-2:2017).”
How do you fill in a PDF form on a Mac?
Preview, the viewer that comes with macOS, fills AcroForm fields. Apple’s guide says: “Click a field in the form, then type your text.” Save, and the values are stored in the fields.
When a PDF has no fields, Apple’s guide offers a workaround: “If a PDF won’t allow you to type directly into the form, you can add a text box instead.” A text box is an annotation placed over the page; it does not change the page’s text either.
For a dynamic XFA form, open it in the app its issuer names. If it shows a notice instead of the form, filling it anywhere else will not work.
Why can’t I change this form field?
| What happens | Usual cause | What to do |
|---|---|---|
| Clicking the field does nothing | The author marked it read-only, or a script locks it | Ask the issuer; the field is meant to stay as filled |
| No fields at all, only lines and labels | The form was printed to PDF or flattened | Fill with a text box, or ask for the fillable version |
| Fields locked after signing | A signature can lock the fields it covers | Ask the sender for a new copy to fill |
| Blank page or a message instead of the form | Dynamic XFA in a viewer that does not render it | Open it in the app the issuer names |
| The field shows one value, prints another | Value and appearance out of step | Regenerate the appearances, or fill the field again in a form viewer |
What does flattening a filled form do?
Flattening draws each field’s appearance into the page content and removes the field. The values become ordinary page text and graphics: they print and display everywhere, and they can no longer be changed as fields. People flatten a form before sending it so that the answers stay as filled and every viewer shows the same thing.
Command-line PDF tools can flatten a filled form on a Mac, and some of them handle form fields specially. Generating the appearances first makes sure the flattened page shows the current values, not stale ones. Keep the fillable original. The guide to flattening a PDF covers the methods and what each one keeps.
What does a PDF text editor do with form fields?
It depends on the editor; here is what PDF Textfix does, since it is a text editor and not a form filler:
- It does not fill forms. Fields are not interactive in its window, and when a document has fields the Issues pane says so: “Form fields can’t be filled in PDF Textfix”, with the advice “Values typed into the form would not be saved. Fill the form in another app, then edit its text here.”
- Field text is locked. A line that belongs to a field is read-only, with the reason: “This text belongs to a form field or a comment, which viewers draw again from its own value, so it cannot be edited here.”
- Dynamic XFA opens read-only. A banner says “This file is a dynamic form, and apps that show such forms ignore edits to its pages.” You can choose Allow Editing, after a confirmation that warns that viewers that render XFA ignore edits to the page text.
- Page text around the fields is editable. A typo in a printed label or a paragraph of the form is ordinary page text, and Save copies the fields and their values as they were, unless the file was damaged; PDF Textfix tells you when it repaired one.
- Signed forms are a special case. A signed form’s signatures no longer verify once you save an edit to it, and PDF Textfix asks before your first edit of a signed or certified file.
If a filled form has been flattened, its answers are page text, and they behave like any other line.
Which route fits which need?
- Type answers into a form: a viewer that supports forms (Preview, which comes with macOS, fills AcroForms), or the app the issuer names for XFA.
- Make the answers permanent: flatten a copy.
- Fix the printed text of the form itself: a text editor, on a form you are entitled to change.
- Fix a value inside a field: fill the field again in a form viewer; an editor’s text tools do not reach it.
The help pages explain how PDF Textfix shows locked lines and the Issues pane. For everything else that stops an edit, see why can’t I edit the text in a PDF? and the form field entry in the glossary.
Questions people ask
Can I turn a non-fillable PDF into a fillable form?
Yes, with a form-authoring tool that adds fields over the page. For a one-off, a text box placed over each blank does the job in Preview, without creating fields.
Why does my filled form look empty in another app?
The value was saved without an updated appearance, and that app draws the stored appearance. Regenerating the appearances, an option some command-line PDF tools have, or saving again from a form viewer usually fixes it.
Can you edit a fillable PDF?
Two ways. Its answers change in the fields, with any form viewer. Its printed text, the labels and paragraphs, changes only with a text editor, which leaves the fields to the form viewer; see what a PDF text editor does with form fields, signed forms included. A flattened copy has no fields left: its answers are page text, which a text editor can correct like any other line.