How to save a diagram smaller than the built-in page size?

[Ming Kin Lai <minglai hotmail com>]

I have a very simple diagram with just 4 text boxes so it is very small in size; it merely occupies mybe one 
fifth of the default A4 paper size.  I want to save it as a PDF so that I can use it in my LaTex document 
(with pdflatex). And Dig exports the diagram in a full A4-size PDF.  I tried to adjust the A4 paper size but 
there is a limit as to how far I can push the bottom margin line up.  So, in general, how can one save a 
diagram in a PDF document just large enough to contain the diagram?                                          
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