If I recall, the PDF you refer to is based on WotC's own Ivid files, which are in their Older Edition downloads section of D&D.  I realistically don't see it as a problem in linking to it, because it is being offered for free anyway, and the text is unchanged from the original.

The problem comes if you attempt to add sidebars from the other books.  That's copyright infringement, and not legal unless you get permission to do so.  Now if you want to make up your own sidebars based upon information gleaned from the other books, that's legal.  The trick is that it has to be in your own words.  Ideas cannot be copyrighted, but words can.