It is better to email a PDF of your presentation than the original PPTX PowerPoint file.
  1. Fonts get rendered correctly, even if the receiving party does not have them installed on their computer. This is especially true now that more and more people are starting to use Macs and are running PowerPoint for Mac, an application that does not allow font embedding. 
  2. Many people open PPTX files on their Blackberry. A PDF is your safest bet that everything renders correctly.
  3. Many people use Gmail as their email system (hiding a gmail address behind what looks like a regular domain). In Gmail, you have the option to view a file rather than downloading and opening it. When you select view, the document gets opened in Google Docs, which is not very good at rendering PowerPoint files (not only fonts go wrong, but entire shapes can go missing)
  4. A PDF opens nicer and cleaner than a PPTX that lands in you the slide edit mode.
  5. PDF file sizes are usually a lot smaller than PPTX file sizes.
The consequence of all this is that you should think twice about using animations in your slides. I switch more and more to just copying a slide and building content page by page, so it will show correctly in PDF format.