Optimize PDF output

@jkridner @RobertCNelson It's not a huge improvement but saved 3MB with pdfcpu.

Using ghostscript we can optimize a lot but,

1. It takes a long time to compress the PDF.
2. It converts TOC to plain text instead of links to specific section.

I tried different optimization options,

1. -dPDFSETTINGS=/screen is not usable as images are all blurred out but I was able to reduce 97MB -> 12MB.
2. -dPDFSETTINGS=/ebook bearable quality but we are still loosing the TOC links to sections reduces 97MB -> 24MB