A PDF form does not contain its answers where you see them. It holds a set of fields with values, and a drawn appearance on the page. So the answers stay editable by whoever opens the file, they can be read straight out without looking at the page, and a field that was cleared or marked invisible can still be carrying what was in it. Flattening draws each answer onto the page for good and takes the fields away.
Drop the PDF here, or
Visible fields are always drawn in and removed, fields that are not on the page are removed without being drawn, left-over objects holding the old answers are discarded, and code attached to the document always goes. Those are the point of it.
A filled-in form is a form, not a statement. Until it is flattened, every answer is a field value that the person you send it to can change, and that anything reading PDFs can pull out without looking at the page. Signing a form and emailing it is not the same as sending what it says.
A field that is not on the page can still hold its answer. A field marked hidden is not drawn anywhere, and one marked not-on-screen is drawn only when the document is printed. Both are still in the file with their contents, and a value typed and then deleted can survive in an earlier revision of the same document. The table above lists every field and what is in it, invisible ones included, which is the part people do not expect to see. Those are removed outright rather than drawn in: burning an invisible answer onto the page would put it in front of the reader instead of taking it away.
Annotations are separate from the page too. A comment, a highlight or a sticky note is an object carrying its author's name and the time it was written. Reviewers' names travel with a document far more often than they mean to. Links are kept by default, because removing those turns a document's own cross-references into dead text.
Some forms cannot be drawn in by anything but a viewer. A file that leaves its appearances to the reader has fields that look empty until one is generated. Those are generated here with a standard font first, and if a particular field still cannot be drawn it is named rather than silently dropped. Look at the result before you send it.
It happens in this tab. A filled-in form usually holds a name, an address, a date of birth and sometimes bank details. Uploading that to a website to make it uneditable would be a strange trade. Turn the network off and this page works exactly as it does now.
What is taken out is taken out of the file, not just off the page. Removing a field from a document usually leaves the old object sitting there unreferenced, still compressed, still readable by anything that reads past the page. After flattening, the result is walked from its catalogue and everything nothing points at is discarded, so an answer that has gone is actually gone from the bytes.
Because a PDF form stays a form. The values are stored as field data and drawn on the page separately; a reader shows the drawing and lets anyone change the value. Flattening removes the fields and leaves the drawing, so there is nothing left to type into.
Yes, until it is flattened. Field values are plain objects in the file, so any library, any command-line tool and any indexing system can list them. That is also how a value in a field marked hidden gets found.
It is removed, and the result says which ones and what was in them. It is not drawn onto the page, because that would take something the document was not showing anybody and print it across the middle of it. If you wanted that answer visible, put it in a visible field before flattening.
It should not. Each field's drawn appearance becomes page content in the same place. The one thing to check is a form that never had appearances drawn, where a standard font is used to make them; open the result and look before sending it.
A drawn or image signature in a field becomes part of the page, which is usually what you want. A cryptographic signature does not: flattening changes the file, so any digital signature over it stops matching. Flatten first and sign afterwards, in that order.
It cannot be opened without the password, and the page says so rather than failing obscurely. Remove the protection in whatever you use to open it, then flatten.