Launch Automator, and, in the Choose a Type for Your Document pane, click on Service, then click Choose. At the top of the window, in the menu following Service Receives Selected, choose PDF Files. Next, click on PDFs in the Library column of Automator, then in the second column, click on Extract PDF Text and drag that to the right-hand section of the Automator window.
Press Command-S to save the service, and, in the dialog box that displays, give it a name, such as PDF to Text. Click on Save.
Check to make sure it's working. Find a PDF file in the Finder, right-click on it, and choose the Services sub-menu from the contextual menu that displays. You should see your service listed there. Also choose Finder -> Services, and check to make sure it shows up in the menu there. You should see it in both places.
Notice that if you select a file that isn't a PDF, the service won't show up; that's because you specified, in the beginning, that the service would only work with PDF files.