Missing fonts need to be installed in order to properly render the fonts used by the prototype creator in ProtoPie Studio.
To make relevant fonts available on an iOS device, send the fonts via messaging apps, email or the cloud. On iOS, select the attached file, tap "Share" and then tap "ProtoPie Player" to make that font available.
Download the relevant fonts on an Android device. Go to Font Manager under Setting, and tap the +button on the upper right corner. Select the newly downloaded font.