Summary
Using the Trace Tool in ArcSite is an excellent way to draw over top of a PDF and trace the outline of existing images/plans. Using the trace tool allows you to add the PDF's lines to ArcSite's canvas so you can take advantage of all the other drawing tools.
How it Works:
When you upload an existing plan as a PDF into ArcSite you can take advantage of the Trace tool. By activating the Trace tool underneath the Edit tools, green lines will appear on your uploaded PDF indicating the lines you can trace over in ArcSite. By simply pressing and dragging your finger where you want to trace, ArcSite will add a line over top of the PDF.
Additional Tools
Underneath the Trace Tool are trace settings, where you can manipulate how sensitive the trace should be, as well a adjusting for centerline or border tracing.
Additionally, you can hide the underlying image by pressing the background icon to show/hide the background image. This gives you the ability to see only the lines you've added to ArcSite while tracing.