A filled-in form is still a form: the values sit in interactive fields that any viewer can change, and some viewers helpfully clear them when printing. Flattening turns those values into ordinary page content.
It's the step you want before emailing a completed application, and it also stops a recipient's reader from re-prompting them to fill in what you've already filled in. It's equally useful for a document you've signed: flattening means the signature is part of the page rather than an object a viewer could select and move.
Flattening tends to make files a little smaller too, since the interactive machinery — the field definitions, their appearance streams and the form dictionary — is discarded once the values are drawn in. On a heavily-fielded government form that can be a noticeable saving.
Type into real form fields, or anywhere on a flat form.
Draw, type, or upload a signature and place it on any page.
Number the pages, with control over position and format.
Add DRAFT, CONFIDENTIAL or a logo across the pages.
Cut away white margins or crop pages down visually.
Combine files into one PDF and set the order before you merge.
Break a document into separate files by range or interval.
Drag pages into a new order, rotate or drop the ones you don't need.
Turn sideways or upside-down pages the right way up.
Pick pages to drop, visually or by range.
Keep only the pages you want as a separate file.
Combine JPG or PNG images into one PDF, with page size control.
Render pages to images at the resolution you choose.
Shrink a PDF with a side-by-side quality preview and a size target.
Add a searchable text layer to a scanned document.
Extract text as plain text or Markdown, in reading order.