Natively there is not a way to trigger a automated report export in Jira Portfolio, However if you have the HTML macro enabled for Confluence, you can use the Portfolio report/share button to get an iframe url snippet you can put into an HTML macro to render the data in a confluence page.
This will allow you to display various Portfolio reports in a Confluence page. Once the page is in Confluence you can use the method covered in This Community Post to do an automated export of the page using a script locally to generate a PDF of the page.
Additional details on generating the iframe can be found here
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