Merge/Combine Electronically Sign PDF

Hi,

I have two SECURED PDF with Document Security. One is SECURED PDF without signature and another one is the same SECURED PDF but have been signed.

Below are Document Security details for both of them:

SECURED PDF - no signature

1. Security Method: Password Security
2. Document Open Password: No
3. Permission Password: Yes
4. Printing: High Resolution
5. Changing the Document: Not Allowed
6. Commenting: Not Allowed
7. Form Field Fill-in or Signing: Not Allowed
8. Document Assembly: Allowed
9. Content Copying: Not Allowed
10. Content Accessibility Enabled: Not Allowed
11. Page Extraction: Not Allowed
12. Encryption Level: 128-bit RC4

SECURED PDF - already signed

1. Security Method: Password Security
2. Document Open Password: No
3. Permission Password: Yes
4. Printing: High Resolution
5. Changing the Document: Not Allowed
6. Commenting: Allowed
7. Form Field Fill-in or Signing: Allowed
8. Document Assembly: Allowed
9. Content Copying: Not Allowed
10. Content Accessibility Enabled: Allowed
11. Page Extraction: Not Allowed
12. Encryption Level: 256-bit AES

My problem is I cannot Merge signed SECURED PDF into another PDF. In the other hand, I can Merge or Combine SECURED PDF without signature in there.

Document Security for Document Assembly that allowing Merge/Combine for both document is already True (Allowed).

Why I cannot Merge it? Is there any method I can use? Is there anything wrong with my Document Security details?

PDF Portfolio cannot be used in this case because we're not going to share the password. It will kill the purpose of having the PDF Secured.