How to flatten a pdf form to lock fields before sending
- Step 1Finish filling the form first — Complete every text field, checkbox, and dropdown in your PDF viewer and save. Flattening is permanent, so nothing should be left to type after this step.
- Step 2Keep a copy of the editable original — Save the unflattened, still-interactive file somewhere safe. If the client comes back with a correction, you edit that copy and re-flatten — you cannot un-flatten the locked one.
- Step 3Drop the filled form onto the tool — Open the PDF Flatten tool and drop your PDF. There is no Process button — flatten starts the moment the file is read.
- Step 4Let the flatten run — The tool regenerates each field's appearance, draws it onto the page, and removes the field. A few seconds for a typical form.
- Step 5Check the values rendered correctly — Open the result preview and confirm the baked-in text matches what you typed — especially long entries and any accented or non-Latin characters (see the edge cases below).
- Step 6Download and send — Download
yourfile.flatten.pdfand attach it to your email. The client sees your values as fixed page content with nothing to click.
What flattening does to each part of the form
The tool runs pdf-lib's AcroForm flatten. This is what survives, what changes, and what it does not touch.
| Element | After flattening | Notes |
|---|---|---|
| Text fields | Baked into static page text | The value you typed is drawn onto the page; the field is deleted |
| Checkboxes & radio buttons | Baked into static marks | The checked/unchecked appearance is drawn; the toggle is gone |
| Dropdowns & list boxes | Baked into the selected value | Only the chosen option remains visible; the list is removed |
| Field borders / shading | Drawn as they appeared | The widget's appearance stream is rendered as-is, including any box outline |
| Comments / sticky notes / highlights | Untouched (still editable) | These are annotations, not form fields — use the Annotation Remover |
| Page text, images, layout | Unchanged | Flatten only operates on the interactive form layer |
Free vs paid limits for flattening
Real limits from the tool's tier configuration. Flatten accepts a single PDF per run.
| Tier | Max file size | Max pages | Files per run |
|---|---|---|---|
| Free | 2 MB | 50 pages | 1 |
| Pro | 50 MB | 500 pages | 1 |
| Pro + Media | 500 MB | 2,000 pages | 1 |
| Developer | 2 GB | 10,000 pages | 1 |
Cookbook
Concrete client-send scenarios and exactly what flattening produces for each.
Quote form with editable prices, locked before sending
You filled a quote template with line-item prices in form fields. Before emailing, you flatten so the client cannot quietly change a figure and forward it as if you quoted it.
Before: Unit price field [ 1,200.00 ] ← clickable, editable After: Unit price 1,200.00 ← static page text, no field Download: quote-acme.flatten.pdf
Confirming what flattened actually means
Open the flattened output in any PDF viewer and try to click a field. If flatten succeeded, there is nothing to select — the values are now part of the page.
Open flattened PDF → click where a field was Result: no cursor, no edit box — it is page content now (In the original, clicking the same spot put you in an editable field.)
Keep the editable master, send the flat copy
The correct two-file workflow. The master stays editable for revisions; the flat copy goes out the door.
onboarding-form.pdf ← MASTER, keep, still editable onboarding-form.flatten.pdf ← SEND THIS, locked If the client needs a change: edit onboarding-form.pdf → re-flatten → new flat copy
Form with a stray comment that survives the flatten
Flatten only removes form fields. A reviewer's sticky note is an annotation, not a field, so it stays. Strip it separately before sending.
Step 1: Annotation Remover → drops the sticky note Step 2: PDF Flatten → locks the filled fields Result: clean, locked, no leftover review markup
No form fields in the file at all
If the PDF has no interactive fields, flatten has nothing to bake. The tool still re-saves the document and hands you a valid PDF — your filled-looking content was probably already static.
Input: flyer.pdf (no AcroForm fields) Flatten: nothing to flatten — re-saves the document Output: flyer.flatten.pdf (visually identical)
Edge cases and what actually happens
Field values use non-WinAnsi characters (CJK, some accents)
may fail silentlyWhen flatten regenerates a field's appearance it uses pdf-lib's default Helvetica, which is a WinAnsi font. A value containing characters outside WinAnsi — Chinese, Japanese, or some accented forms — can throw during appearance generation. The tool catches that error and re-saves the document, so the field may come out NOT flattened (still editable). Always open the result and confirm the values are baked in before sending.
Flatten is irreversible
By designOnce flattened, the interactive fields are deleted from the file — there is no un-flatten. If you might need to amend a value, keep the editable original. This is the single most common regret with form flattening.
Sticky notes and highlights remain
PreservedFlatten operates on AcroForm fields only. Comments, sticky notes, and highlights are annotations and survive untouched. Run the Annotation Remover first if the form was reviewed before you send it.
PDF is larger than 2 MB or has more than 50 pages on Free
blocked on freeFree flattening caps at 2 MB and 50 pages. A bigger form is blocked with an upgrade prompt; Pro raises the ceiling to 50 MB / 500 pages. Most single client forms sit well under the free limit.
The form is an XFA (LiveCycle) form
not supportedDynamic XFA forms — common from Adobe LiveCycle / Designer — are not standard AcroForm fields, and this tool does not render or flatten the XFA layer. Open such a form in Adobe Acrobat, which can convert XFA to a static PDF, then flatten there.
Password-protected (encrypted) form
decrypts on saveThe tool loads encrypted PDFs by ignoring the encryption, but it saves the output WITHOUT a password. A flattened, decrypted file is fine to send to a client, but do not treat the output as still-protected. If you need protection back, re-apply it with the Password Protect tool.
Empty fields you forgot to fill
ExpectedFlatten bakes the current state. An empty field becomes a blank space (or an empty box outline) with no way for the client to type into it. Fill everything before flattening, or the client will be stuck with a blank they cannot complete.
Field had a coloured border or grey shading
PreservedThe field's widget appearance — including a box outline or background fill — is drawn onto the page as part of flattening, so the flattened result may still show the box. This is normal and matches how the field looked; it is just no longer interactive.
Frequently asked questions
Can the client still edit a flattened form?
No. Flattening deletes the interactive fields and bakes their values into the page as static content. In a normal PDF viewer there is nothing to click. A determined person with a full PDF editor could still edit the underlying page like any document, but the form-fill convenience — click, retype, done — is gone.
Do I lose anything visually when I flatten?
No. Flatten regenerates each field's appearance and draws it exactly where the field was, so the page looks the same. The only difference is that clicking no longer puts you in an edit box. If a value renders wrong (rare — usually a non-WinAnsi character issue), the edge cases section explains why.
Is there a Process button or any options?
No. Flatten is a zero-option tool. You drop the file and it runs immediately — there is nothing to configure, which means there is no way to accidentally leave a field unlocked through a setting. You just download the result.
What does the downloaded file get named?
It takes your original name with a .flatten.pdf suffix — for example quote-acme.pdf becomes quote-acme.flatten.pdf. That makes it easy to keep the editable master and the locked copy side by side without overwriting.
Will my form data be uploaded anywhere?
No. Flattening runs entirely in your browser using pdf-lib. The filled form and the values inside it never leave your device. Only an anonymous usage counter is recorded when you are signed in, and that contains no file content.
Can I un-flatten if the client wants a change?
No — flattening is permanent. That is why the recommended workflow is to keep the editable original, send the flattened copy, and if a change is needed, edit the original and flatten again. Treat the flat file as a one-way export.
Does flattening remove comments or highlights from a reviewer?
No. Those are annotations, not form fields, and flatten leaves them in place. If your form was reviewed and you do not want the markup going to the client, run the Annotation Remover before flattening.
My form has signature lines — will flattening keep them?
If the signature is a drawn image or a filled appearance, flatten bakes it in visually. If it is a real cryptographic digital signature, flattening modifies the file and breaks the signature's validity — see the flatten signed contracts guide for the archiving workflow and verify first with the Signature Verifier.
How big a form can I flatten for free?
Up to 2 MB and 50 pages on the free tier. That comfortably covers single client forms. Larger or multi-form bundles need Pro (50 MB / 500 pages). The tool flattens one PDF per run regardless of tier.
Why would a client see empty boxes instead of my filled form?
Some lightweight viewers and email previewers render interactive form fields poorly — showing empty boxes or missing values. Flattening converts those values to plain page content, which every viewer draws correctly. That is one of the best reasons to flatten before sending.
What happens if my PDF has no form fields?
Nothing to flatten — the tool simply re-saves the document and gives you a valid PDF back. If your form looked filled but had no real fields, the content was already static, so the output is effectively identical.
Should I also compress the form before sending?
If the flattened file is large or you want a tidy attachment, run it through Lossless Compress afterwards. Flatten and compress are separate steps — flatten locks the data, compress shrinks the file.
Privacy first
All PDF processing runs locally in your browser using PDF-lib and pdf.js. No file is ever uploaded — only metadata counters are saved for signed-in dashboard stats.