Here is how I do:
After importing the PDF or other drawing:
Is your import tied to the point of origin?
Is your imported drawing parallel with your red/green axis (your Blue Plane)…?
other wise rotate it so it lines up.
Start with the tape measure,
dubble-click on red axis to set a guideline
then click once, to drag a line parallel to red axis
and set it on other side of “Paper” with a second click.
Repeat for green axis.
Draw a rectangle on top,
bigger than the drawing you imported…
your drawing will disappear…
but choose “Face-style” X.Ray…
and it will show through.
To see the import from the top…
from menu, select: Camera/Standard view/Top
Delete guide lines…
drag a new from edge or axis into / on to the import,
where there is a important line/feature, you know the Length of.
Draw a line along this object.
Use the tape measue, to measure it…
when you click the second time, (look at the VCB box)
and type the correct measurement for this line.
Sketchup will ask, if you want to size the model?
Choose Yes, and now your import will be to scale.
Make a layer for the import and move the layer here( via entity info)
Right click the import and “Lock” it.
Now you can turn the import on/off by turning the layer on/off.
Save the project.
and only then,
you start drawing lines and rectangles…
on your Giant rectangle (your “paper”)
Switching x-ray on and off, will help keep your own drawing in check…
to se if you have closed boxes (rectangles) as walls etc.
When rooms, walls and other features are finished…
you can switch to iso mode and start pulling up.
It is a good idea, to make a scene for the top view and a scene for the iso view.
Please let me know if you can use this…
if you need further help…