Translate Java to Python -- signing strings with PEM certificate files

wish help you to fix your issue First, certificates are not "SSL certificates". They are x.509 certificates, some of which can be used for SSL/TLS authentication. If you have such certificate, you can't use it for other purposes including PDF signing.
Next, .cer file is a public part of the certificate, it doesn't include a private key. You need a certificate and a private key to sign anything.

I wish this helpful for you You can get free account at PingOne from Ping Identity.
After that you will be provided with certificate and meta information to use PingOne as identity provider.

it helps some times If the corporation has its own CA root cert, which yes had to be pushed to every client/relier by some means such as GPO or installing all systems (or maybe their JREs?) from a customized image, then your approach is almost correct:
generate keypair and CSR send CSR to corporate CA, they send you back cert combine cert with keypair in keystore and use keystore

wish of those help I finally solved it - here is the story:
When I bought the certificate, I had to collect it by navigating to a website address that I received by email. There, the certificate was automatically installed into the truststore of my browser (Firefox).

it helps some times Most likely those code signing certificates have an extKeyUsage (2 5 29 37) extension with value codeSigning (1 3 6 1 5 5 7 3 3) and probably also commercialCodeSigning (1 3 6 1 4 1 311 2 1 22) but no values allowing other uses. (At least that is the case for a code signing certificate I have at hands here.)
RFC 5280 specifies for the Extended Key Usage extension that if the extension is present, then the certificate MUST only be used for one of the purposes indicated.