Is it possible to do?
Looks like the logo is a vector drawing directly embedded inside of the pdfreporter config file.
You can hide it, but as far as I know you can't replace it. Quoting from the docs: "The reportIncludeSplunkLogo parameter for alert_actions.conf controls whether the Splunk logo appears in the PDF footers. It is set to 1 (true) by default. If you don't want to see the Splunk logo in your PDFs, set the value to 0 (false)."