Measuring PDF margins, etc. with Preview? I would like to measure the size of the margins and other distances in a PDF without having to print it out. I would like the results in "as printed" units (e.g. inches or cm once printed)

Is it possible to measure PDF margins or other distances with Preview? 

If not is there another (hopefully free) application that can?

Update: 
I see at this TeX stack exchange post some options for windoze and linux. Any thing like this for OSX?
Found it!

In Preview, 


open the Inspector (Command-I) 
Select the Crop and Rotate tab, whose icon looks like a small ruler. Change the units as required
Now use the select area tool and the corresponding (printed) dimensions will be shown in the Inspector.


As an alternative, you can also download Free Ruler, which displays a nice ruler on screen, though requires care in both setting the ruler DPI and ensuring that the PDF is viewed at 100% size.