What flattening does to a form
Each box in a fillable form carries the value you typed and a small drawing of how that value should look. When the tool flattens the form, it takes that drawing and places it into the page content at exactly the spot where the box was, at the same size and angle, then deletes the box and the form behind it. The result looks the same as before, but there is nothing left to click or type into: the name you typed is now printed on the page like the rest of the text. Because the drawing is copied as it is, the fonts are kept, so the text stays sharp, can be selected and copied, and search still finds it. Ticked check boxes and selected radio buttons keep their tick, drop-down lists keep the chosen option, and empty fields simply disappear.
Forms filled in other programs, and Arabic values
Not every program stores that drawing properly. Some office programs and phone apps save the typed value but leave the drawing empty, and ask the reader to draw it each time it opens the file. Flattened as it is, such a field would come out blank, which is how values get lost. This tool checks every field: when the value is there but its drawing is empty or missing, it draws the value again first. Latin text is redrawn as real text with a standard font. Arabic, Urdu and other scripts need a font the file does not have, so those values are drawn by your browser, with the letters correctly joined and right to left, and placed on the page as a sharp picture; the summary tells you how many values were handled this way. In our tests with a form exported from LibreOffice with Arabic values and empty field drawings, every value appeared on the flattened page.
Comments, stamps and highlights
Comments, highlights, underlines, drawings, text boxes and stamps are drawn into the page the same way, using the look the commenting program saved. Simple highlights, underlines, boxes, circles, lines and ink drawings that were saved without a look are drawn from their outline and colour. A sticky note keeps its small icon, but the text in its pop-up window is removed with the note, because a page cannot show a pop-up; copy anything you need from the notes first. Comments marked as hidden are removed. Links to websites and to other pages stay clickable, since they do not change how the page looks.
Layers
Some PDFs, often drawings, maps, brochures and files exported from design programs, put parts of the page on layers that the reader can switch on and off. "Keep only visible layers" removes the content of layers that are switched off when the file opens and joins the rest into the page, so what you see is exactly what everyone else will see and print. "Show every layer" makes every layer visible, including ones that were off. In both cases the layer list is removed, so readers no longer offer to switch anything. "Don't change layers" leaves them as they are.
Selectable text or images, and what flattening is not
Flattening with selectable text is the right choice for almost every job: the file stays small, text can still be searched and read aloud by screen readers, and it prints sharply. "Flatten everything to images" draws each page at 150 or 300 dpi and saves only the pictures, which suits printers and portals that only want fixed pages, but the text can no longer be selected and the file grows. Flattening is not security. Anyone with a PDF editor can still add text on top of a flattened page, and hidden information elsewhere in the file is not removed. To remove private data for good, use Redact PDF; to control who can open the file, use Protect PDF. A digitally signed file stops verifying once it is flattened, so keep the signed original. Files up to 100 MB work in most browsers.