All SSL Certificates comes in .pem format

However, to import it into server such as Windows Server you may convert it into 1 file (.pfx) so that customer can important all certificate with 1 file with the password we have created with the file

Prepare the followings :
1. signed certificates (save as signed.cer or signed.crt)
2. private key file (save as private.key)
3. intermediary certificate (save as intermediate.cer or intermediate.crt)
4. root certificate (save as root.cer or root.crt)
5. pfx password (**Create your own password)

You may do it via web by upload the files you have prepared earlier :
https://www.sslshopper.com/ssl-converter.html

The output file will be in .pfx format, anyone with the password can import into the server or extract it back to .pem format.