PDF Editing Issues

How to Fix Fill and Sign Unknown Error in PDF Documents

Huda Qurayshi Oct 18, 2026 869 views

Question details

The user is encountering an 'Unknown Error' message preventing them from successfully filling out form fields or applying a signature to a PDF file.

How to Fix Fill and Sign Unknown Error in PDF Documents

Product: PDF Editor
Device & OS: not provided
Scenario: Attempting to complete a digital form or insert a digital signature into a PDF document.
Observed behavior: The application fails to process the input or save the signature, instead displaying a generic 'Unknown Error' prompt that halts the workflow.

Before you start

Ensure your PDF document is not restricted by a password and that you have sufficient permissions to edit or sign it. Saving a backup copy of your original PDF to your local drive before attempting repairs is highly recommended.

Solution 1Recommended

Flatten the PDF by Printing as a New Document

If the file contains corrupted form fields or incompatible metadata, printing it to a new PDF can flatten the elements and allow for successful signing.

Complex interactive forms created in different software versions can sometimes trigger unknown errors. By flattening the document, you convert interactive fields into standard text and graphics, which generally resolves signature application issues.

Open the Print Dialog

Open the problematic PDF in your current PDF viewer and click on 'File' > 'Print', or press Ctrl + P (Windows) or Cmd + P (Mac).

Select a PDF Printer

In the Printer dropdown menu, select 'Microsoft Print to PDF' or 'WPS PDF' as your chosen printer.

Save the Flattened File

Click 'Print' and choose a destination to save the new PDF file. Open this newly saved file and try using the Fill and Sign tool again.

Flatten the PDF by Printing as a New Document

Flattening Complete: The new file will no longer have fillable text fields, so make sure you fill out any required text information before performing this step to sign it.

Solution 2

Verify Document Permissions and Security

PDFs can be locked by their authors to prevent modifications, which often surfaces as an unknown error when trying to force a signature.

Solution 3

Update or Reinstall Your PDF Editor

An outdated PDF reader can cause unexpected glitches with the Fill and Sign feature due to software bugs that have since been patched.

Seamlessly Fill and Sign PDFs with WPS Office

WPS Office provides a robust, error-free PDF editor that lets you easily fill forms, add text, and insert legally binding signatures without annoying compatibility issues.

1. Open the PDF: Launch WPS Office and open your target PDF file directly from the home screen.
2. Access the Fill & Sign tool: Navigate to the 'Fill & Sign' tab located on the top ribbon of the PDF interface.
3. Add your signature: Click 'Sign', create or choose your existing signature, and click anywhere on the document to place it securely.

Easily fill out interactive and non-interactive PDF forms with dedicated tools.Create, insert, and manage multiple digital signatures with high security.High compatibility with standard PDF formats including Adobe Acrobat files.Free, lightweight, and works seamlessly across Windows, Mac, and mobile platforms.

microsoft office alternative - wps office

Frequently Asked Questions

Why is my PDF signature not saving after applying it?

This usually happens if the PDF is set to Read-Only, or if the file directory you are working in requires administrator privileges to save changes. Try moving or saving a copy of the file to your Desktop before signing it.

Can a corrupted PDF cause an unknown error during signing?

Yes, damaged metadata or improperly formatted interactive fields can trigger an unknown error in your PDF software. Flattening the PDF by printing it as a new file or using a reliable editor like WPS PDF can often bypass this issue.

How do I remove security from a PDF so I can sign it?

If you know the permissions password, you can go to the document's Security settings and select 'No Security' to remove the restriction. If you do not have the password, you must request an unlocked version from the document's original creator.

PDF Troubleshooting