Here is the usual reason for scaled printing to fail when first creating a PDF from SketchUp by printing or exporting. Basically it has nothing to do with SketchUp,
When you print from PDF to paper Acrobat or Adobe Reader defaults to “Shrink to printable area”. Even when the PDF file paper size is the same as the paper in your printer, Acrobat will add additional margins around the printout and it will no longer be to scale. Page scaling in Acrobat/Reader must be set to None.
Anssi