Flatten PDF

How the Flatten PDF works

A filled PDF form is two things layered together: the printed page, and a set of live fields that float on top of it. The same goes for comments, highlights and stamps added in a reader. Each layer can be edited, moved, hidden or lost depending on the app that opens the file. Flattening merges the top layer into the page, so the file looks the same everywhere and nothing can be changed by clicking on it.

Why flatten a PDF

• Sending a finished form. A signed application, invoice or tax form should not be editable by the next person who opens it. Flattened fields cannot be retyped.
• Printing and merging. Some printers and older merge tools drop live fields entirely, so a form prints blank. Flattened values are ordinary page content and always print.
• Mobile and browser viewers. Many lightweight viewers show form fields poorly or not at all, and some show comments as tiny icons. After flattening, every viewer shows the same page.
• Archiving. A record that will be kept for years should not depend on a reader's form support.
• Sharing a reviewed draft. Flattening comments turns highlights, sticky notes and stamps into marks on the page, so a reviewer's markup cannot be edited or deleted by the recipient.

How Flatten PDF works

The file is opened with pdf-lib, an open-source PDF library, inside your browser.

Form fields only reads every field in the form. Each field's visible appearance, the drawing a reader shows for its current value, is painted onto its page at the field's position. The field and the form structure are then removed. Text boxes, checkboxes, radio buttons, dropdowns and list boxes all keep their current look.

Form fields and comments also goes through every annotation on every page. Annotations that have an appearance, which includes highlights, underlines, ink drawings, shapes, stamps and free-text notes, are drawn onto the page in the same way and then removed. Pop-up note windows and hidden annotations have no place on the printed page, so they are removed without being drawn. Links are always kept, because a link is not a mark and flattening it would just break it.

Tips and pitfalls

Keep the original. Flattening cannot be undone. Save the editable form first in case a value needs to change later.

Check the fields are filled the way you want. A field shows whatever value it holds when it is flattened. If a reader saved a value without updating its appearance, open and save the form in a form filler first so the appearance matches.

Digital signatures. A cryptographic signature covers the exact bytes of the file. Any change, flattening included, breaks it. Flatten before signing, never after.

Sticky-note text is lost. A note's comment text lives in its pop-up, not on the page. Only its icon is drawn. If the wording matters, copy it out or use a reader's "summarise comments" export.

Hidden fields stay hidden. A field marked as hidden or no-print has nothing to draw, so it simply disappears.

How to read the result

The stats show how many pages the file has, how many form fields were flattened, how many comments were drawn onto pages and how many pop-ups or hidden notes were removed, plus the file sizes before and after. If the file has nothing to flatten you get a message instead of an unchanged copy.

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