You need to delete the PrimoPDF settings XML file, which can be found in your Application Data folder.
- Navigate to %AppData%\PrimoPDF
- Delete PrimoSet.xml. You could back it up first, but I wouldn’t say it’s important.
After doing so, upon opening PrimoPDF again, the settings file will be re-created. Note that your previous PrimoPDF settings will have been lost, so make sure you set them correctly again.
Credit goes to itsupport.vizada.com for pointing in the right direction (though with slightly outdated instructions).