Bluebeam is not CAD software, which is exactly what you're describing happens in, for example, AutoCAD.
When I want to draw things to scale, step one is set the scale, then draw a dimension and edit the dimension to set the value to each dimension you need, say 5', 10', 20', etc. Then you can snap a normal line, rectangle, circle, or anything else, to those setup dimensions. Every length needs to be measured and set via some dimension and then you can snap to each one as required to draw what you need to scale. It's also helpful to get used to combining markups and rotating them (as you cant just "rotate" a line, but you can if you group it with anything, even just a temporary line).
It's not perfect, but this is still by FAR the best PDF viewer I've ever used.