Is there an easy way to check if a PDF being inserted to paperspace is drawn to scale? Like if I were to take an actual scale and measure a doorway that I know is 36 inches wide..how can I check that?

I don't normally overlay PDFs to paper space... usually they're drawings or copies of as builts that I want in model space.  But even if you insert it with a scale factor things can often be off because of paper shrinkage one way or the other.  So I always do two things:
1.  Draw a line on the PDF on a long horizontal.  Use ORTHO mode to make sure it is truly horizontal.  Then draw another line (ortho off) on the PDF on a line that is also supposed to be horizontal.  If there is any rotation in the PDF rotate it back to the horizontal using the angle found between the two lines you've drawn.
 
2.  Once the horizontal is established draw a line on a known distance on the PDF and check the AutoCAD length.  Example.  You have a dimension on the PDF of 55.65 ft.  The AutoCAD line measures 61.25 ft.  Your PDF needs to be scaled downward 0.90857.
 
When you take care of all this your PDF will be both properly scaled and rotated.  If you now create a viewport in paperspace that includes the PDF it will be properly scaled there as well.
 
Dave.