Lyman - first, you should always send the certificate to your client. If they ever need it, they would already have it. Certification includes the certificate. I have run across a couple races that claim "certified" courses, but they were unable to produce the certificate. They never had one, and that is the point. Always send a cert to the client.
Now, about file size. How was your 980 kb PDF created? Did you scan it? Was it color, or monochrome?
My observation has been that black-and-white PDFs convert to small png files, but PDF files produced from color scans don't convert to small png files. That is what I am trying to discover - if others can take a PDF created from a color scan, and make a small png file from that PDF file.