How can I convert a PFX certificate file for use with Apache on a linux server?

With OpenSSL you can convert pfx to Apache compatible format with next commands:

openssl pkcs12 -in domain.pfx -clcerts -nokeys -out domain.cer
openssl pkcs12 -in domain.pfx -nocerts -nodes  -out domain.key   

First command extracts public key to domain.cer.
Second command extracts private key to domain.key.

Update your Apache configuration file with:

<VirtualHost 192.168.0.1:443>
 ...
 SSLEngine on
 SSLCertificateFile /path/to/domain.cer
 SSLCertificateKeyFile /path/to/domain.key
 ...
</VirtualHost>