PowerPoint slides have missing characters in the PDF version

I'm using PowerPoint to compose art for banners.  The page size is 33" x 21" and each banner has a single graphic and two layers of text. 

I upload the PPT file to be converted and SOME of the text is just NOT THERE in the PDF version.  It's true for all 4 slides I converted, and generally the earliest and largest type on each slide is most likely to be missing.

Stuff just doesn't work?