I have found that adding a known dimension border around your DXF file first helps. Example. Say the border is 30"x 50" make a notation on the file as to the size of border for printing reference. When you start a new PDF file set it's size to custom and set it for the Border size plus a 1/2" ( 30.5" x 50.5 "). Then import the DXF file as 1:1 inch scale. Save as a PDF. Done!