I am writing a php web application which needs to open pdf files in the browser with Adobe Reader. Some people have multiple versions of the Adobe Reader plugin installed to their browsers. Adobe Reader 10.1.3.23 seems to be causing problems with opening pdfs for some people, Reader 9.5 seems to still work fine though. Is there a way to progarmatically force a clients browser to use an older version of the Adobe Reader plugin if the client has multiple versions of the Adobe plugin installed on their browser?
No. You don't have this kind of control over a user's browser. And that's a good thing because it could be used to exploit Adobe Reader bugs which seem to litter previous versions of the software.