Many investors will scroll rapidly with the page down key through a presentation that is attached to an email. They will skip the cover letter in the email body (too much text), they will skip the dense summary page one of the presentation (too much text) and will continue to scroll down your slides, stopping at pictures (newspaper readers read the caption of a photo before the headline of the article).
I have seen horribly looking presentations that pass the PGDN test. When scrolling rapidly through the pages you actually get what the author wants to say.
I have seen very sophisticated, professional-looking presentations that completely fail the PGDN test (many were written by consulting firms and investment banks).
To your own PGDN test before cold-emailing your pitch.