The pencil, highlighter, underline, strikeout, and line annotation tools all include a wrist guard option.
Tap the wrist guard icon (this looks like a finger pointing) in the active tool's ribbon in the upper right of your screen to activate this feature.
Use the tab at the top right of the wrist guard to adjust the size of the overlay as needed. You can then rest your wrist or palm at the bottom portion of your screen while annotating at the top.
The wrist guard will stay open in the same position (on a document-specific basis) while using any of the tools mentioned above until deactivated.
You can, however, set it to only stay open for the pencil tool via App Settings > Annotation > Auto-Display Wrist Guard for Pencil Only."
Please note: The wrist guard cannot be moved to the top of the screen at this time.