In my template file, I have several Design Layers always ready. The 'bottom' layer I call trace and I use that layer to import files I need to, well, trace. I this instance I would go to File>Import>Import PDF.
I do not see anything in your PDF that I can scale, so first I might hand draw something in there that I could scale. Fir this reason, I always suggest adding a Scale Bar object in any Sheet Layer Viewport I might export.
Once you have something you know the dimension of in your file (you may have a known dimension, possibly the bottom boundary of the site), select the PDF and go to Modify>Scale Objects. Select the second option to scale by distance. Click on the Dimension line next to the Current Distance data field and that will return you to the PDF. Draw a line across the distance you know, release and that will return you to the Dialog. Enter the proper distance in the second stat field, make sure that "entire drawing" is not checked and click OK.
There are some occasions where you will want to scale all, but be aware of the option.
Then on a new design layer above the Trace Layer begin to work. I generally work in the "Show/Snap" others mode. How you structure your Design Layers will depend on your workflow, I work in entertainment, so I have each now file with Scenic, lighting, sound layers and add as needed to isolate objects.
If the PDF was a vector based document, you can "ungroup" and access some of the internal line work, that can be very valuable.
In any event, this should get you started.