Links to PDF's can be made to target specific places in the document by appending a number/pound (#) sign to the end of the URL, followed by "page=[number]:

Example: target page 10

http://server.com/title.pdf#page=10

Example: Target a named destination:

http://server.com/title.pdf#destination_name

Thanks to the PowerPoint FAQ for this tip.
It is also possible to control what happens when the document opens - which view, what zoom, etc. See Link to PDFs from HTML -- The PowerPoint FAQ for more details. Note: this works most consistently with absolute links, and not at all with file-system links (c:\folder\) Relative links that target specific pages within a pdf may work in some browsers, but some people will merely be directed to the first page of the pdf.