The original .png files are each about 80kb so 320 total? My PDF is undoubtably smaller or not?
As a techy type... I prefer facts ... but as a general nerd of sorts... I'm also not beyond making wild guess's for the fun of it either. So some number guessing it is... I'll get back on topic in a separate post, but this is the fun one for me.
I can only surmise that whatever method you used to make the PDF, the images are smaller ( ie resized it ) or did some sort of conversion in the process. I have nothing that will take apart a PDF to know what image format it is using internally, I can only COPY from a PDF and then paste the image copy... then my image program will save as whatever file type it supports. So that would be basically the same process most people would also use. I can give details about the pasted image, the format I chose and it's size both for the image dimensions and space. I can say I'm using Adobe Reader as my PDF reader, then Paint.net as the image editor to save as a PNG file... I convert using XnView because of the number of image formats it supports.
Take the image listing the firmware version options... Saved as a PNG, it's pixel dimensions are 641x629 pixels using 24 bit color. The size on disk is 47,.29 KB. Saving it as a JPG, its size on disk is 30.23 KB. If you are the nerdy type and like guessing at the rest... You can compare the pixel sizes I have to the originals first to see if they are the same or not. The next will be with the approx number you gave for disk size.
You said the original was a PNG... assuming if the dimensions were not changed from what you started with, then we can safely assume they converted the image to one that does compress the image file in order to save some space. Notice the size differences I got converting to the JPG, you'd get it was about 64% compression. Now lets compare the disk sizes of what you listed of around 80 KB to my 47 KB and we see it's just under 60% smaller than what you listed... give or take since we aren't using exact numbers now. So a very close to the same compression amounts. So we can then assume they either used JPG as the compression format to embed the image or at least an image format that comes close to the same compression level.
If you go back to what I mentioned about JPG having different compression levels... I typically will use ( and did this time ) an 80% compression algorithm. There is a big difference in 80% vs the 64% I ended up with but we have to take into account what it does to compress... The image is mostly white across the whole image with only a few bits of text and graphics here an there. Because of all that white... we ended up with a better amount of compression in total in the end. We can also apply the same thought process to the original... and assume they used a similar compression amount give or take a bit.
You can get a little more accurate numbers if you wish since you have the originals and better compare the numbers. I just used approximates since we are guessing at a lot here anyway. If you wish... look at the dimensions ( pixels ) as well as the actually original disk size if you want to dive a bit deeper into it. As mentioned, it won't change what I surmised it will only make the numbers more accurate.