If you have a PDF document on the web and you want to draw the attention of your friend to a particular page in the document, you can send him a link to the page and also tell him the page number to look at.
But there is a more convenient alternative. You can actually add a reference to the page when you send the link which will open the particular page instead of the first page. You can do this by adding the parameter page=<pagenum> at the end of the link.
Here is a live example.
Clicking on the above link will open up the fourth page automatically. There are more such parameters that you can use. For instance, you could add the parameter zoom=<zoomvalue> to open the pdf at a particular zoom level.
For more parameters, check out Parameters for Opening PDF Files (PDF) in the Acrobat SDK documentation set.