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How to increase space between bibliography items?

I try this. But it gives me errors mentioned that \bibitemsep is undefined.

Thanks for the reply and modifying!

I am using Adobe Acrobat Pro Extended 9.2.
I have attached screenshots of my problem.
The pdf file in the screenshots is generated using the latex code I have attached, I am not sure is it due to the acrobat version I am using?

Hi, in my pdf documents,
it is ordered by i,ii,iii(with abstract etc)
then started by 1,2,3,...(the contents)
just like the those normal books we have seen.

I have seen some pdf files able to display the page number directly matched the page number specified, i.e. it could display i,ii,iii,.., 1,2 ...