Files with .png images create huge .pdf's

Question

I use .png images in PowerPoint presentations and these seem to create huge .pdf files. (...don't have this problem using .jpg images) For example, I have a 48 slide presentation that has a small .png image in the "slide master" so it will show up on every page. The PowerPoint file is 2.3MB, however the .pdf of it is 33.3MB, making it WAY too big to email, which was my objective.
Any ideas on how to reduce the size of .pdf's which contain .png images?
...note that I "compressed" the .png images inside of PowerPoint prior to creating the .pdf, but this made no impact.
Thanks,
pm

Answer 1

>I have a 48 slide presentation that has a small .png image in the "slide
>master" so it will show up on every page.
PowerPoint only has to store one instance of the image even thought it is
displayed on multiple slides. But Acrobat can't do the same, the image has
to be stored for every page that it appears on, so that would explain the
larger file size.
Acrobat is supposed to be able to optimize the storage of common background
images (at least that's the message I see when I do a "Save as..."), but I
have yet to see it to work. Maybe it depends on the application the orginal
file is created in and the PDF conversion method used.
Adding the image into a PDF file later as a watermark instead may be a way
to keep the size down (then it really only does store the object once),
though I believe that watermark images will appear in front of everything
else on the page (not sure if that's a problem for you, it depends on
whether any text or objects in the presentation ever appear in front of the
image).
What are the dimensions and resolution of the image in PowerPoint? You may
be able to go as low as 75 dpi and not notice a difference, it depends on
whether the image contains any areas of high contrast or text.