Why does the measurements on my pdf file don’t make on what on lightburn?
Without knowing anything more about the PDF or the measurements, it’s hard to say.
Is one in metric and the other in inches?
Perhaps the PDF uses a combination of scaling that LightBurn doesn’t handle?
Tell us more and somebody may recognize the problem.
Thanks for the quick response. Here is the scenerio. I have a ship plan that I photocopied as a pdf file. It is a part of the ship hull. I measure the width and height in mm. When I iimport into lightburn as a svg and define the same measurements as the pdf to cut. The cut size are different from the pdf file. What shall I do to make the resulting cut wood the same size as the pdf?
Then you say…
How did you convert from PDF to SVG?
No matter, determine the ratio between the PDF and what you got, and use that as a multiplier in the width or height box.
Although the laser motors on a Sculpfun should be calibrated already, have you verified that a 1 inch square drawn in LightBurn produces a 1 inch square engraved on some cardboard? If not, then the axes need calibrating.
Are you taking the laser kerf into account? If not, measure the kerf and either embiggen the parts with an offset or apply a kerf offset to the entire layer.
And, as @MikeyH points out, maybe the photocopy / print / scan / trace / whatever sequence has done some scale damage along the way. Checking that will depend on what went on, but consumer-grade printers are notorious for nonlinear distortions we never notice on a printed page. 
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Wow! I never knew this can be complicating! Must be another way of printing or have someone convert pdf to ai. Or svg in 100%
Welcome to the World of Laser Hobby!
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