File Size - .pdf vs .ppt

[Verbal Kint]

I have a pdf file which is 25MB even after compressing the pictures in it via the option in Power Point. When I print the file as .pdf format I notice the file size drops to 3MB. Does anyone know why there is such a big difference in file size between the .ppt file and the .pdf I created from the .ppt? Our work email system only allows us to send a 10MB file and this causes a problem when trying to send large files.

Thanks