You can export an image of a document with a callout (and also an image of the document before the callout is applied if you want to show a before-and-after type of effect). The exported image will also include any highlights or other annotations you may have made.
The best way to do this for exporting to PowerPoint is to view the document in TrialPad and press the Snapshot button at the bottom of the Preview window (the button looks like a camera) and name it "Document X". Then create the callout on the document, with any highlights you may want, and press the Snapshot button again, and name this file "Document X Callout". Both of these Snapshots will be saved to a newly created Snapshots folder in the main directory of TrialPad. (For a larger callout effect you can collapse the Document List before taking the Snapshot which will make the callout text appear larger.)
To export these two images to use in PowerPoint, open the new Snapshots folder, press the Select button, select the images by tapping on them (or press the Select All button if you've created a number of images), then press Edit, and then Export. In the resulting popover select a destination which can be iCloud Drive, On my iPad, or a connected USB drive (which might be the best choice if you need to move the images to a desktop computer).