TrialPad can import and play almost all audio and videos that are compatible with the iPad or Mac it is installed on. If 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 compatible format. It has various presets to make the conversion process easy, with the Apple 1080p30 Surround option being a good choice for most situations (see attached screenshot).
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.