My son bought a used chevrolet astra 2000 van and wants me to download
the owners manual from the internet.How can this be done?

I assume you mean a Chevy Astro van. Manuals are usually not avaliable
for download. However, getting a manual for a common American vehicle
is easy. Two companies print consumer service manuals:

Haynes: http://www.haynes.com
The ISBN for the current Chevy Astro/GMC Safari manual is 1563924773

Chilton: http://www.chiltonsonline.com
Chilton does not appear to print a current manual for the Astro,
however, the manual for the 1985-1996 Astro should be compatable.

A search at amazon.com for Astro manual or the ISBN for the Hanyes
book should get you what you want. These manuals cover all basic and
some advanced maintenance. Good luck!

The original owners manual would not be available for download.  The
haynes and chilton manuals are great for repair.  If you want repair
information online try http://www.alldatadiy.com If you want the
original owners manual i would recommend Ebay.  Searching for "2000
astro manual" currently shows one up for bid.  You may also want to
give your local Chevy dealership's parts department.