No, I'm not misunderstanding what you are doing. That was just a polite way of saying I don't know why you are posting this in the PHP forum since it has nothing to do with PHP.
I'm not a graphics expert, so I don't know the comparison between the gif and png. My understanding is that png has a much better compression algorithm for it, so an image that's identical in both png and gif should be smaller in png format. Personally I'd probably use a pdf. I don't know the pdf structure aside from the header, but I understand it's searchable which means I can write a program to extract the information from it. I'm far too lazy to do data entry. After that I may convert it down to a png.
header('HTTP/1.1 420 Enhance Your Calm');