Maybe this is just me, but what makes a huge difference when looking at slides or PDFs is to see that the one who made it put effort in. Not necessarily themselves, it’s perfectly okay to have a designer or layout person set it up. The absence of visible effort results in a loss of perceived value.
When speaking of presentations, the choice never really boils down to fitting in by uniformity or leveraging the message by standing out, because the slides are just visual aids. They are not the presentation. What you do is. Just make sure the slides do not work against you. Practical example: Trying to sell innovation by displaying standardized conservatism is not helping.
Regarding downloadable PDFs the issue is slightly different. I prefer vibrant, fun-to-read, comic-style formats with compact text and sidebars, stuff that keeps the eyes moving. Entertain your readers. If you want to sell, don’t make a book. Trust your gut feeling. If you have none, hand the job to someone who has. Your readers will thank you.