One approach could be to use the OverlayPdfDocument action in the Community Commons module on the Marketplace. This allows you to overlay one PDF over another. In your case, you could have a PDF containing a blank certificate, and you then overlay the data in your generated document over the top to give the result you need.
https://docs.mendix.com/appstore/modules/community-commons-function-library/#43-files
Hope this helps.