Flattening bakes editable elements — filled form fields, comments, drawing annotations — into the static page content of a PDF. After flattening, viewers see everything but can't edit the fields or delete the comments. Useful before sharing a signed contract, submitting a completed form, or archiving marked-up docs where you don't want reviewers to re-open comments.
What gets flattened
AcroForm and XFA form fields, sticky-note comments, highlighter/underline/strikethrough annotations, freehand drawings, text boxes, stamps, and layers (OCGs). Everything becomes non-editable page content.
Why flatten
Guarantees that reviewers see exactly what you see — no accidental annotation edits, no missing annotations in viewers that don't render them. Common workflow: fill a form, flatten, send. The recipient can't accidentally clear fields.
Reversibility
Flattening is one-way. Once flattened, form field structure is gone — you can't extract the values back into an editable form. Keep an unflattened copy if you might need to re-edit.