If you received the PDF file as an attachment to an email, then - if it's not too large - the iPad's Mail app will open it from within the app itself. If it's a very large attachment, it may tell you that it's not been entirely downloaded and you'll have to touch the icon of the attachment to complete the download.
You can read the PDF file in Mail, if you want to, but most users prefer to move the PDF file to another compatible app - such as iBooks or GoodReader - as the previous poster suggested. To do this, when Mail is displaying the PDF file, touch the page and a menu bar will appear briefly at the top of the page offering you the option to open the PDF file in whatever compatible apps you have available on your iPad.
If you downloaded the PDF file in Safari, then there's a similar process. Like Mail, Safari can display the PDF file too but it's usually easier to open it in either iBooks or GoodReader and the way to do that is the same as with Mail, touch the page and select the app you want to open the PDF file in.
Tim