[OS X TeX] Cut a piece of a pdf

[Alain Schremmer]

Is it possible to cut and keep just a piece of a pdf, say the top  
third of a page?

I have googled a bit but did not find anything other than cutting out  
a page from a multiple page document or cropping the margins, none of  
which is what I want.

The only way I know how to do it is by grabbing a selection with  
Preview but the result is not too good.

Hopeful regards
--schremmer