It seems that the PDF that results from doing a "File" > "Export" > "PDF Export" is scaled to about 98% of what it should be.
I am making a ruling for an optical test and I have constructed it in Model space with metric units. Length precision is 0.0000 (Format > Unit System > Length > Precision 0.0000; Units Scale millimeters). I have drawn the ruling lines in full scale (so a polyline with width 1.0000 should be 1.0000 mm wide). In Paper space, I have set up a sheet for the output (print scale 1:1) and placed a viewport in Paper space with a scale of 1:1. I created a non-printing "viewports" layer and moved the viewport border there so the border won't show up in the output.
If I print or plot that page, the scale is 100% (I checked lines 800 mm apart and they are 800 mm center-to-center). I have a long, direct-read glass reticle for precise measurements so I am sure the measurements are accurate.
If I use File > Export > PDF Export, then the resulting PDF file has the lines only 783.929 mm apart center-to-center which is a 98% scale factor. I am exporting to a paper size that matches the sheet size set in Paper space.
If I set the scale inside the Viewport in Paper space to 1.0205, the direct plot is off (of course) but the PDF Export result is almost right on. I verified the final PDF by importing it into Inkscape and measured 800.015 mm across the same two lines, center to center, which is very close to what it should be. When I print that PDF file using Adobe Reader, the scale is also about right.
--> I am wondering why PDF Export scaling is just a bit "off".
Some other experiments:
If I change the "custom" paper size for the PDF Export, the scaling changes but the error remains. If I export to a custom sheet that is double the size of the Paper space page, the resulting graphic is scaled to 1.96 times the original (which is 2 times 0.98). From that I conclude that the PDF Export does a "fit to page". The error is slight, perhaps a slight loss of precision in a scale factor calculation somewhere within the export function. If I change the scale factor in the Viewport, the scale of the PDF Export follows directly (with the same small error).
You have to have a printer or plotter or PDF writer that can be set for the target PDF sheet size. In my case, this is an odd size (850x150mm), so I had to define that paper size in the Windows Print Server (Windows Start Button > Devices and Printers > select any printer > "Print server properties" > Forms > (may need to hit the "Change Forms" permission button depending on Windows security settings) > "Create a new form".
