Description:
I cannot sign an existing PDF document - my OpenPGP key (managed with GnuPGP) - is not shown. However, it works perfectly fine to sign a newly created (opened in "write mode") document with the same key.

Steps to Reproduce:
0. Have some OpenGPG private key generated and available via GnuPG
1. File → Digital Signature → Sign existing PDF
2. Select and open PDF (can be previously signed or just exported from Writer)
3. Click "Sign Document" button
4. Click "Sign Document..." button
5. Press Cancel (or ESC) on a prompt note for NSS Certificate DB password


Actual Results:
My certificate (an OpenPGP key) is not shown on the "Select Certificate" list.

Expected Results:
I see my OpenPGP key (with signing capabilities).


Reproducible: Always


User Profile Reset: No


OpenGL enabled: Yes

Additional Info:
When I create a new document in Write/Calc/Draw and use:
File → Digital Signature → Sign Document...

I see my OpenPGP key and can successfully sign that document. I just don't work with existing PDF files (even those just exported using LibreOffice).

I makes it problematic to (among others) sign the PDF already signed by other parties.

Software information:
Version: 6.4.6.2
Build ID: 6.4.6.2-3.fc32
CPU threads: 12; OS: Linux 5.8; UI render: default; VCL: gtk3; 
Locale: pl-PL (en_US.UTF-8); UI-Language: en-US
Calc: threaded