Why are Illustrator EPS open paths with aligned end points closed (joined) when creating a PDF?

Hi,

I have an Illustrator CS6 file with open paths and aligned end points.

When I create a PDF using Acrobat the end points are automatically closed (joined) and a mitre is applied which results in an awkward looking sharp edge to the line.

A good example of this would.be the letter N made of 3 separate paths.

If I create a PDF from Illustrator the paths are left open, et voila, no mitred corners!

How can I avoid this behaviour when using Acrobat to generate the PDF?