Currently Key Docs can only be a single page, or the whole document. The workaround for a situation like this would be to view the document in the Documents tab in TrialPad, press the Share button and press Share Entire Document > Print. In the Printer Options press Range and select the page range that you want to be a separate document (e.g. 3 through 5), then when you go back to the preview you will see all the pages shaded except pages 3 through 5. Now do a pinch-to-zoom on one of the non-shaded preview pages to open up those pages as a new PDF, and then press the Share button at the top and choose an option that is easiest for you to get the new document back into TrialPad, e.g. Save to Files, Mail (to email it back to yourself), etc.
If you also have DocReviewPad another workaround would be to view the document in TrialPad, press the Share button, select Open Document In... and choose DocReviewPad. Once you have the document in DocReviewPad you can use the Split or Extract tools to break up the document and send it back to TrialPad.