For the digital signature of your documents, you need a signing certificate in .p12 format (public and private key).
You can either upload an existing one or generate a new one.

If you already have a signing certificate, upload it as /app/data/resources/cert.p12 using the File Manager.
If the certificate has a passphrase, set it in /app/data/env file:

Restart the app for the changes to take effect.

To create a self-signed certificate, open a web terminal and run the following commands:

cd /run
# generate private key
openssl genrsa -out private.key 2048
# generate self-signed certificate valid for 10 years
openssl req -new -x509 -key private.key -out certificate.crt -days 3650
# create p12 certificate. password is optional
openssl pkcs12 -export -out cert.p12 -inkey private.key -in certificate.crt -legacy
# move cert into documenso
mv /run/cert.p12 /app/data/resources/cert.p12
chown cloudron:cloudron /app/data/resources/cert.p12

If you set an optional password, open File Manager and put passphrase into /app/data/env file:

Restart the app for changes to take effect.