Music, videos, photos, documents? They are all handled differently
For example, in Apple's iWorks app Pages - you can delete the files locally - on the iPad.
However, if you've synched say photos onto your iPad then you need to delete them on your PC and then resync - you can't delete those synched photos locally. It's the same with music and videos - if they've been synched with your PC then you need to delete them on the PC and then resync
As the last poster suggested, if you go to the iPad's native 'Settings' app, click on 'Safari' you'll see the option to 'Clear History' and 'Clear Cache'.