You've filled in a PDF form and you want to send it off, but you'd rather nobody could change your answers afterwards. Or someone has asked for a "flattened" copy of a document and you're not sure what that means. Flattening takes everything that's interactive in a PDF, such as form fields, and merges it into the page itself, so it becomes a fixed part of the document. It's a short step with a long-lasting effect.
What "flatten" means
A PDF can have several layers of content on top of each other: the page itself, form fields you can type into, comments, and signatures. While those layers exist separately, they can be changed. Flattening presses them together into one layer. A filled-in text box becomes ordinary text on the page; a checked checkbox becomes a tick. After that, the answers can't be edited in a PDF reader, which is exactly what you want for a form you've finished.
Two modes
Flatten form fields keeps everything as text and vectors. The page stays sharp, searchable and small, and only the interactive fields become fixed. This is the right choice for completed forms. The tool tells you how many fields it found, and if there are none it says so instead of silently doing nothing.
Flatten everything goes further: it redraws every page as a picture. After that nothing can be edited, selected or copied, including comments, highlights and anything else sitting on top of the page. You choose the resolution, from small files at 100 dpi to print quality at 300. It's the strongest option, at the cost of a bigger file and no searchable text.
When you'd flatten a PDF
- Finished forms you're about to send, so answers can't be altered afterwards.
- Documents with comments or highlights that you'd like to merge into the page, for example before sharing with someone using a simple viewer that doesn't show them.
- Before printing, when a printer or print shop drops form fields or prints them oddly.
- Before signing and sending, so the signature sits on a fixed page.
- Archiving documents in a form that looks the same in every viewer.
A worked example
You've downloaded a tax form with fillable fields and typed your details into it. Before sending, you open it here, see "Found 24 form fields", leave the mode on Flatten form fields, and download. The new file looks identical, but if you open it in a reader and click a field, nothing happens: your answers are now part of the page. If you also want to add a signature, do that first with Sign PDF, then flatten, so everything is locked together.
Before you flatten
Flattening is a one-way street, so keep a copy of the editable version. Check your answers, because afterwards you can't fix a typo without going back to the original. If the form doesn't have fillable fields and you need to type onto it, use Add Text to PDF first. If parts of the page contain information you don't want to share, use Redact PDF; flattening locks content but doesn't hide it.
After flattening
If you chose Flatten everything and the file grew, Compress PDF reduces it again, or you can lower the resolution and try again. A flattened document is also a good moment to add page numbers and a watermark. For a black-and-white archival copy, PDF to Grayscale can do the same job in a different style, and the metadata editor will clean the file's hidden properties.
What flattening does not do
It doesn't encrypt the file or stop someone from copying or re-typing the content. A flattened PDF with selectable text can still be copied from; a fully flattened picture can still be put through OCR software. If you need to prevent changes to a legal document, flattening is a sensible first step, but for strong guarantees use a qualified digital signature from a trusted provider. Flattening also doesn't remove the hidden metadata inside the file.
Troubleshooting
If the tool says there are no form fields, your PDF probably isn't a fillable form; use "Flatten everything" if you want to lock the pages. If some fields are still editable in a reader, they may belong to a feature this tool can't flatten, such as a signature field. If the result looks different, use a higher resolution when flattening everything. Everything happens in your browser, so the document is never uploaded.
A note on signatures
If a form contains a digital signature field from a certificate provider, flattening can invalidate the signature, because the signed content changes. That's normal. If you only have an ordinary image of your handwritten signature, as made with Sign PDF, flattening is safe and sensible. When a document must keep a certified signature, don't flatten it; send the signed original as it is.