Flatten PDF

A PDF page has two layers of content. The first is the page itself: text, pictures and drawings that make up the printed document. The second is a set of interactive objects that sit on top of it: form fields with their typed values, check boxes, comments, highlights, sticky notes, stamps, drawn signatures and digital signature fields. These objects are stored separately, so a reader can show, hide, edit or remove them.

Flattening moves the second layer into the first. Each field value, comment or stamp is drawn into the page content exactly where it appeared, and the interactive object is removed. Afterwards the document looks the same, but there is nothing left to click, type into or delete.

What to flatten decides which objects are affected. Everything handles all of them at once and is the usual choice for a final copy. Form fields turns only the filled-in fields into page content and leaves comments interactive, which suits a completed form that still has review notes attached. Comments & markup fixes highlights, notes and stamps in place but keeps the fields editable. Signatures flattens only signature fields, for a copy meant to be printed or read.

The reasons for flattening come down to two things: making a document look right everywhere, and making it final. Interactive objects are drawn by the viewer, and not every viewer draws them the same way. Some phone apps, email previews and older programs ignore form values or comments, so a carefully filled-in form arrives looking blank. Once the values are part of the page, every viewer and printer shows them.

Finality matters for anything that is submitted, filed or archived. An application form, a signed agreement with its annotations, or an inspection report with stamped approvals should not be quietly editable after it is sent. Flattening removes the easy way to change those values in a PDF reader.

Flattening is not the same as protecting. A flattened page can still be edited with full PDF editing software, the way any PDF page can. What disappears is the easy path: typing over a value in a form field or deleting a comment in a free reader. For a document that must not be altered at all, combine flattening with a password and keep the original as proof of what was sent.

The list above sums up Vector, the default. It is the right method for nearly all forms and reviewed documents: the result looks exactly like the original, reads well on any screen and prints crisply.

Image takes a stricter route. Each page is rendered as a picture, and the new PDF holds only those pictures. Everything that was not visible on the page is gone: hidden layers, text under redaction boxes, earlier revisions, form data and metadata inside page objects. That makes it the safer choice when a document has been redacted by drawing boxes over text, or when you must be certain that nothing beyond the visible page survives. The price is that text becomes part of an image, so it cannot be selected, searched or read aloud by a screen reader unless text recognition is run again.

Merge layers applies to PDFs with optional content groups, which some drawing and design programs use to let readers switch parts of a page on and off. Merging combines them, so a layer that is hidden now cannot be turned back on. Keep links and Keep bookmarks leave clickable links and the bookmark panel as they were, and Remove metadata deletes author, title and software details from the result.

The resolution setting matters only when Flatten as is set to Image. It decides how many pixels are drawn for each inch of the page, and so how sharp the result looks when someone zooms in or prints it. Higher values give cleaner text but larger files, because each page picture gets bigger in both directions.

The default of 200 dpi is a good balance for forms and letters. Go to 300 dpi for documents with small type, such as contracts with footnotes, and only use the top of the range for plans or drawings with very fine lines. If an image-flattened file is too large to send, compress it afterwards rather than lowering the resolution so far that the text becomes hard to read.

Scans make a useful comparison. A scanned page is already a picture, so image flattening at a similar resolution changes little about how it looks. A page typed in a word processor, on the other hand, loses its crisp vector text, which is why Vector stays the default for anything that was not scanned.

Flattening usually happens just before a document leaves your hands. Fill in the form, review and resolve the comments, then flatten and send. If the recipient needs several documents in one upload, flatten each one and combine them with Merge PDF. The merge tool also has a flatten option of its own, which avoids a known problem: fields with the same name in different files would otherwise share a single value in the merged document.

Flattening does not make a document private. A flattened PDF opens for anyone who has it. To control who can open it, add a password afterwards with Protect PDF. To share a single page as a picture instead of a document, PDF to Image is the simpler option.

Signatures need care. A digital signature proves that a document has not changed since it was signed. Flattening the signature keeps its visible appearance, but removes the data that software uses to check it, so the copy no longer proves anything on its own. Keep the signed, unflattened original as the record and flatten copies only for printing or reading.

When a batch holds several files, they all get the same settings and come back as separate PDFs. Run a form that needs Image flattening in its own batch if the rest should stay as vector.