TrialPad can import and play almost all videos that are compatible with the iPad. If your video files seem unusually large, or you are having issues with the compatibility of a video file, you can use the free Handbrake desktop program which is available for macOS and Windows (https://handbrake.fr) to convert the video into a smaller size in a compatible format.
Handbrake has various presets to make the conversion process easy, with the Apple 1080p30 Surround option being a good choice for most situations.
If you’re viewing the video in the Dropbox app on your iPad, and it plays back without an issue, it still may not be in a compatible file format to work on the iPad. The reason for this is that Dropbox transcodes the video as it’s being streamed to make it viewable on any device. If you access Dropbox from within TrialPad you will only see the compatible file types.