Stroke turns into fill when saving...

Hello,

Sometimes when I save a file in PDF with some strokes thickness they become fill. I notice that because my cutting machine didn't recognize the solid color with fill. It needs a stroke to cut. 

It's strange because if you re-opened the PDF (obviously after saving) in Illustrator the shape remains in stroke, but if you select that shape in Acrobat (I do that with Pitstop plugin) you see the fill instead of the stroke....

Did you notice that?

I attached here a file. If I open it with Illustrator I see the red contour as a stroke, if you select the same object in Acrobat you should see as a fill.

Thanks

Francesco