You first need an app on your iPad that supports PDF documents so that you have somewhere to copy it to.
If you have iBooks on your iPad then you can add the PDF to the Books part of your computer's iTunes library via File > Add To Library, and you can then sync it to the iBooks app on your iPad.
Syncing and viewing PDFs in iBooks : http://support.apple.com/kb/HT4227
If you have a different app that you want to copy it to then how you do so will depend upon what the app supports e.g. the file sharing section at the bottom of the iPad's Apps tab when connected to your computer's iTunes, via your wifi network, Dropbox, email.