Convert your PFX/P12 Certificate file to a seperate certificate and keyfile.
openssl pkcs12 -in [yourfile.pfx/p12] -nocerts -out [keyfile-encrypted.key]
This extracts the private key from the pfx/p12 file. Once entered you need to type in the import password of the pfx/p12 file. This is the password that you used to protect your keypair when you created your pfx/p12 file. Once you entered the import password OpenSSL requests you to type in another password, twice! This new password will protect your .key file.
Extract the certificate:
openssl pkcs12 -in [yourfile.pfx/p12] -clcerts -nokeys -out [certificate.crt]
If your device/software requires an unecrypted .key file use the following command to remove the passphrase from the private key:
openssl rsa -in [keyfile-encrypted.key] -out [keyfile-decrypted.key]
Enter the new password that you created in step 1.
Convert your private key to PEM format. You can do so with the following command:
openssl rsa -in [keyfile-encrypted.key] -outform PEM -out [keyfile-encrypted-pem.key]