The iPad will not play flash based videos. So if those sites are streaming flash encoded videos the iPad won't natively play it but there are browsers in the app store that will play flash video.
how well it plays are based more on the internet speed than anything else.
As to large PDF's, it should...but there have also been some that have had issues opening large files, so nothing concrete there. You'll want a PDF reader app, of which there are several. There are also some video player apps you can look into.