It really depends on the method you use to get the PDF onto the iPad. If you are using iTunes, you can scroll down to the button of the Apps tab and then drag and drop PDFs into the Pages app. Then, you open the Pages app and the PDF should be there, all ready for editing.
Or, you can e-mail the PDF to yourself. Then, from within the Mail app, press on the PDF and you should get an "Open In..." popup. Scroll the list you are offered and one of the apps you can select should be Pages. Select it and the PDF will automatically open in Pages.
Hope this helps.
Marilyn