Here's how to scale a PDF or PNG based on a reference length:
Upload your PDF or PNG and add the file to the object list in SkyView.
Activate the Scaling tool in the toolbar.
Select the PDF or image in the object list that you want to scale.
The right panel for the scaling tool will update, and under Scaling Mode at the top of the panel, choose the option Reference Length.
You now need to place two reference points by clicking on the drawing in the scene. Left-click in the drawing and choose Place point A. The point will then have a blue marker in the drawing, and you'll see its N and E coordinates in the right panel under the section Reference Points.
Click a second reference point in the scene with a left-click and choose Place point B. The point will then have a green marker in the drawing, and you'll see its N and E coordinates in the right panel under the section Reference Points.
Under Reference Length in the right panel, enter the distance between the start and end points in meters.
Execute the scaling by Confirming.