Vector image in a .pptx exported to PDF: how can I avoid quality loss?

I hope someone can help me fix this.

I created a round icon using Illustrator, saved it as a PNG and inserted it into a .pptx file. So far, so good.

Now when I save my .pptx document as a .pdf, the icon suffers a major quality loss. Even when I zoom in on the .pdf, the icon is not a perfect circle anymore and can become blurry, which just looks messy.

When creating the PNG from Illustrator, I've tried saving the icon in various sizes, to no avail. I've also tried tweaking the resolution using Photoshop (from 72 to 150 then 300 ppi), but still can't get to a satisfactory result.

Has any one of you ever encountered this issue and found a solution for it? Thanks a lot for your time and help.