At the top of the page you'll see the heading "Browse" . Mouse over it and choose "Downloads" . In that folder you will see on the right a list of the manuals available. The Honda service manuals can be found under "Other"
Here is the link to the file in that folder you are looking for. It covers both the 2 wheel drive TRX300 as well as the 4x4 TRX300FW models.