Hi there!
In reference to https://support.atlassian.com/confluence-cloud/docs/customize-exports-to-pdf/, I added a simple code in the header and footer section under Look and Feel -> Global PDF Export.
<div>
<img src="/download/attachments/229456/new_footer.jpg" />
<div>Test</div>
</div>
I did not define any additional styles in the PDF stylesheet.
When performing the export, the PDF does not contain either the footer or the header.