Flatten a PDF: troubleshooting common problems

Fixes for the most common problems when flattening a PDF at /app/flatten. The tool runs entirely in your browser, so none of these issues involve a server — everything below happens on your device.

"This PDF is password-protected, so it can't be opened to flatten it"

Most "protected" government forms (like the USCIS I-765) aren't actually password-locked — they only carry permission restrictions, and you can open them in any viewer without typing anything. Flatten handles these automatically: when you drop one, AttachKit removes the restriction encryption right on your device and shows an "Unlocked this protected PDF on your device — nothing was uploaded" notice, then flattening proceeds normally. Your file never leaves the browser during this step.

The only files that are still blocked are PDFs that require a password just to open. For those, Flatten shows a password-protected message with a link to the Unlock tool, where you can enter the password and remove it first, then come back and flatten the unlocked copy.

"This PDF has no fillable form fields, so there was nothing to flatten"

Cause: Flatten works on interactive (AcroForm) form fields. If the PDF has none, there's nothing to bake in — it's already "flat". Common cases:

• A scanned paper form — the boxes are just part of the page image, not interactive fields.
• A form that was "filled" in another app by placing text annotations on top instead of typing into real fields.
• A plain document (letter, report, invoice) that was never a form.

1. Open the file in Fill to check whether it has interactive fields at all.
2. If it's a scanned form, there are no fields to lock — the page image already is the final content, so you can send it as-is.
3. If your goal is to stop other kinds of changes rather than lock fields, see the next section.

The downloaded file can still be edited

Cause: flattening removes interactive form fields — it is not encryption. Annotations and edits made in a full PDF editor are not blocked; no flattening tool can stop a determined editor from altering a PDF.

1. Confirm the fields themselves are gone: open the …-flattened.pdf copy and click where a field used to be — nothing should become editable or focusable.
2. To discourage further changes, add a password with Protect.
3. To make later changes detectable instead of merely difficult, sign the file with Sign — anyone can then check it on the verify page, and any alteration breaks verification.
4. To actually remove sensitive content (not just lock it visually), use Redact — flattening deletes nothing.

"Couldn't flatten this PDF — it may be corrupted or in an unsupported format"

Cause: the file isn't a PDF the engine can parse — typically a truncated download, a renamed non-PDF file, or a damaged file.

1. Re-download or re-export the file from wherever it came from, then try again.
2. Open it in another PDF viewer; if it fails there too, the file is damaged at the source.
3. If a viewer can open it, use that viewer's "Print to PDF" to produce a clean copy, then flatten that copy.

Some fields are still fillable after flattening

Cause: rare, malformed forms (for example a field that references a missing font) can fail partway through. Flatten finishes best-effort and still returns a file instead of crashing, so the odd field can survive.

1. Open the downloaded copy in Fill to see which fields remain.
2. Re-export the form from the app that created it (or "Print to PDF" a fresh copy), then flatten the fresh copy.
3. If it keeps happening, let us know via the link below — a description of how the form was made helps.

The "Fill it →" button after download does nothing

Cause: the cross-tool handoff stashes the file in browser storage (never on a server). In a private/incognito window that storage can be blocked, and the tool shows "Couldn't open it in Fill — download it and drop it into Fill instead."

1. Use the downloaded …-flattened.pdf and drop it into Fill yourself.
2. Or repeat the flatten in a normal (non-private) window and click Fill it → again.

Still stuck?

Contact support and include what the error panel said plus roughly how the PDF was created (scanned, exported, filled in another app). Don't paste anything confidential — your file never reaches us, so a description is what helps.