Set the movie and image formats for Keynote presentations on iPhone
Keynote keeps movies and images in their original format, regardless of whether or not the format can be viewed on older computers and devices. You can change the optimization settings if you want to make sure videos and images using newer formats—for example, High Efficiency Video Encoding (HEVC), High Efficiency Image Codec (HEIC), or Apple ProRes—can be viewed on any computer or device.
Optimize video and image formats
When you add HEVC, HEIC, and ProRes media files to your presentation, Keynote can convert them into H.264, JPEG, and PNG so they can be viewed on older Mac, iPhone, and iPad devices.
Go to the Keynote app on your iPhone.
Open a presentation, tap , scroll down, then tap Settings.
Turn on Optimize Media for Older Devices.
This setting applies only to movies and images you add from now on, and not to any existing media in your presentation.