well if the pdf file is on the same server it's easier to do, otherwise you have to retrieve the contents which can become server intensive (especially since some pdfs can be really large). Let's assume the file is on the same server, you can try using the inbuilt pdf tools (see here
http://php.net/pdf). The easiest way I can think of is if the pdf file is indexed by google, it shows a link that says 'view as html' - you could just retrieve that page and then you have it already converted.
Most of the inbuilt php features are intended for creating pdfs.