Fillable PDFs collect input. Flattened PDFs lock it down. The choice depends on whether you are still collecting answers or are done.
Here is the practical difference.
Fillable PDF
Form fields are interactive. Recipients can type, check boxes, sign. Useful for intake forms, applications and anything that collects input. Built in form authoring tools (Acrobat, some Flint tooling).
Flattened PDF
Form fields are baked into the page. No more editing. Useful when the form is complete and you want to archive or send the result without further changes.
Common workflow
Send fillable form to recipient. Recipient fills and signs via sign-pdf. Recipient (or you) flattens the result before archiving. Two-step pattern, well established.
Best for…
Fillable while collecting. Flat once complete and archival.
FAQ
Can I unflatten a PDF?
Generally no — flattening is one-way. Re-author from source if needed.
Does signing flatten automatically?
Some tools flatten the signature field; others keep the form editable. Flint's sign-pdf lets you choose.
Why flatten?
Prevents recipients accidentally editing a finished form.
Collect with fillable, archive with flat. Flint handles both.