This is another of the posts which is largely to remind me of how to do something. In this case, when I publish a book, I need to convert the PNG of the cover into a PDF.
I usually end ip trying a few services before finding the one which works every time; that service is Zamzar.
I’ve used it to convert a number of PNGs to PDFs now and also to convert a few other files as well. I find Zamzar to be consistently accurate and reliable for converting files.