Skip to content
  • There are no suggestions because the search field is empty.

Activation Error: Invalid Serial Number

When you activate Nitro PDF Pro for Windows with a license file (.lic), you can receive an Invalid serial number error.

A Nitro PDF Pro license file contains a 400-character or longer alphanumeric string that Nitro PDF Pro uses for activation.

Causes of the Invalid serial number error

The following issues can cause this error:

  • You entered the license file name instead of the license file content
  • You copied only part of the license file content
  • You installed a Nitro PDF Pro version that does not match the license file version

For example, a license file for Nitro PDF Pro 14 cannot activate Nitro PDF Pro 26.

Resolve the Invalid serial number error

Confirm that the installed Nitro PDF Pro version matches the version associated with the license file.

To verify the installed version:

  1. Open Nitro PDF Pro
  2. Select the Help tab
  3. Select About Nitro PDF Pro

The version number appears in the dialog box, for example:

  • 26.xx.x.xxx
  • 14.xx.x.xxx

To verify the license version:

  • Review the purchase email from Nitro, or
  • Contact your Nitro administrator

The purchase email includes:

  • The Nitro PDF Pro version
  • A download link for the correct installer

If the installed version does not match the license file version:

  1. Uninstall Nitro PDF Pro
  2. Install the version linked in the Nitro purchase email

Use the Upload License option to load the full contents of the license file automatically:

  1. In the Nitro activation window, select Upload License
  2. In File Explorer, locate the .lic file
  3. Select the license file
    Note: The .lic file is attached to the Nitro delivery email
  4. Complete the remaining fields in the License Information dialog box
  5. Select the agreement checkbox
  6. Select OK

After the activation is completed successfully, Nitro PDF Pro enables all licensed features.

Last updated: June 2026