使用证书和私钥与iText 5签署PDF文档

I have a certificate which is used for tls/SSL in my web server. The certificate was issued by a trusted authority and I am having the RSA private key and x509 certificate for the same. The key usage of the certificate says that it can be used for digital signature. Now i need to digitally sign PDF using that certificate, but in all examples, they are asking for a pkcs12 file which I don't have. Is there is any solution that i can sign the pdf using that key and cer or i can generate the pkcs12 using any openssl command?I have a certificate which is used for tls/SSL