Replace dummy 'certificate.pdf' with the PDF file of your certificate design. Make sure that the design does not have name of the participant, which will be written while generating the e-certificate.

Another option is to peruse the Apple Vintage Software collection at Archive.org which may have image files of older system restore disks and other older system software, just beware that archive.org ...

Open src/main/resources/application.properties file and change the property file.upload-dir to the path where you want the uploaded files to be stored.