Filled PDF form: lock it before you send it
Pick the mode that locks your form without wrecking the file, check the result and know what flattening leaves open.
To lock a filled-in PDF form, flatten it: each field's value becomes part of the page and the field disappears, so nobody can retype it from the form. Flatten PDF does this in your browser, with two modes worth telling apart before you choose, and the form is never uploaded. That matters because forms carry personal details such as names, addresses and ID numbers.
What does flattening actually do?
A PDF form stores each field as a separate object sitting over the page. As long as it's a field, whoever receives the file can change its value. Flattening draws the value onto the page and deletes the field. Other layers, like comments, highlights and stamps, are handled differently depending on the mode.
| Mode | What it locks | What changes in the file |
|---|---|---|
| Forms | The filled form fields | Almost nothing: text stays selectable and the size barely moves |
| Everything (rasterize) | Fields, comments, highlights and stamps | Each page becomes an image, so text is no longer selectable |
Forms is the default and usually the right pick. Everything is the safest, but it changes the file and costs you the text layer.
Printing to PDF vs flattening
A common workaround is to "print" the form to PDF from a viewer, which redraws every page and so produces a flat copy. It works, but it depends on the viewer, it may rasterize or drop things without telling you, and you can't see what was locked. A dedicated tool shows how many fields and annotations it found and how many it fixed, which is the difference when a form gets rejected for a blank box.
How to flatten, step by step
- Open Flatten PDF and drop the file (up to 500 MB). The tool counts pages, form fields and annotations.
- Choose the mode. If the PDF has no fields, Forms mode says so and offers to switch to Everything. If it has annotations that aren't fields, you get a note that Forms mode leaves them alone.
- In Everything mode, pick the image quality: 150 DPI for screen and email, or 300 DPI for print, which weighs more.
- Run it. You get a summary: how many fields were flattened, or how many pages were rasterized and at what resolution, plus the size before and after.
- Download the file, which ends in
-aplanado.pdf.
What to check before you send it
Open the result in the PDF reader and scroll through. Two things can go wrong, and the tool flags both.
- Blank fields. A field can have a value while the PDF stores no appearance for it, and then Forms mode may fail to draw it. This tends to happen with emoji, Cyrillic or CJK characters. The tool names the affected fields and offers to rerun in Everything mode, which does draw them.
- Very large pages. If a page is too big for the browser to draw at the chosen resolution, its resolution is lowered and you're told.
Sending it to a printer or print shop? Everything at 300 DPI is the logical choice. If the recipient needs to copy or search the text, stay on Forms.
What flattening doesn't do
- It isn't a digital signature. Locking fields doesn't prove who filled the document, and someone with a PDF editor can still change content. To add a visible signature, see sign a PDF on iPhone or Android, and for what it counts as, is a drawn PDF signature legally binding?
- It adds no password. To limit who can open it, use Protect PDF, and read what a PDF password actually protects first.
- It doesn't strip metadata or attachments. A flattened PDF can still carry an author, the software used and embedded files. That's what Sanitize PDF is for, covered in hidden data in PDFs.
A sensible order of work
- Fill in every field and check the values.
- Flatten the form.
- Add a signature with Sign PDF or page numbers if needed.
- Check the result in the reader.
- If it's too heavy for email, shrink it first.
Frequently asked questions
How do I make a filled PDF form uneditable?
Flatten it: each field's value is drawn onto the page and the field is removed. Whoever receives it can no longer change what you filled in from the form itself. It doesn't stop someone from editing the PDF with an advanced editor.
What's the difference between Forms mode and Everything mode?
Forms locks only the fields and keeps text selectable, barely changing the size. Everything turns each page into an image with forms and annotations included, locking comments, highlights and stamps too, but text can no longer be selected, searched or copied.
Does flattening reduce quality?
Not in Forms mode: text stays vector and sharp. In Everything mode each page is an image at 150 or 300 DPI, so sharpness depends on that resolution and the file size can change.
Can I undo flattening?
Not on the resulting file. Keep a copy of the original PDF with editable fields in case you need to correct something later.
Does flattening lock signatures and stamps?
Comments, highlights and stamps are only locked in Everything mode. Forms mode doesn't touch them.
Is my form uploaded to a server?
No. It's processed in your browser and the file downloads from your device. You can check the Network tab in developer tools: analytics requests show up, but no POST or PUT the size of your file.
Lock your PDF form
Flatten just the fields or everything, then download the locked PDF. Runs locally, free, no signup.
Open Flatten PDF →Related tools
- Flatten PDF: lock form fields or the whole page, with a before-and-after summary.
- Sign PDF: add your drawn signature to the flattened document.
- Sanitize PDF: remove attachments, JavaScript and metadata before sending.
- Protect PDF: add a password to limit who can open it.
You might also like: how to reorder, delete and duplicate PDF pages and what PDF metadata reveals about you.