Sometimes after uploading a PowerPoint (PPT/PPTX) or PDF file (converted from PowerPoint or Keynote), your slides won't look right in the system. The easy fix is to simply export your slides as PNGs or JPGs, then upload the images directly (detailed instructions for Mac and PC, below).
Here's an example of what can go wrong. Slides with gradient colors and non-standard fonts can get distorted when they're finally uploaded, as you can see by comparing the below two images.
Original Slide As it Appears in PowerPoint
Incorrectly Converted Slide
Obviously, the background was incorrectly converted in the above example, and the fonts are also incorrect. Here's how to fix it.
To Fix: Export Your Slides as PNG or JPG Files
PowerPoint Users: Click on File > Save As (or, depending on your version, select to "Save as Pictures" or "Export"), and under the Format section, scroll down to select PNG or JPG. A folder will get created on your computer and inside that folder, you'll see the exported images.
Keynote Users: Click on File > Export To > Images and follow the instructions.
The final step is to go back to the Slide Decks page, and upload the exported PNGs or JPGs.
Note that you can upload all of the images at once by clicking the CTRL key on your PC and then clicking on the "a" key (CTRL+A). On your mac, hold down the COMMAND and "a" key (COMMAND+A).
For the Curious: Why Don't Fonts and Gradients Transition Perfectly to the Slides System?
The short answer is because Microsoft, Apple, Adobe, Google, and other big tech companies purchase licenses to fonts from companies called type foundries, and they don't have the rights to sublicense them. They might purchase other design elements, including colors, gradients, and patterns, from other companies, and the terms of what they can and can't do with those elements can be messy.
Even different versions of PowerPoint will render fonts and colors differently depending on the version of PowerPoint you're using and your platform (e.g., Mac, Windows, Linux).
The bottom line is the best way to preserve your original appearance is to export from the original program, because that program will create an image that's an exact replica of your original slide.