General Sudoku Sites
There are many Sudoku sites on the Web. A good starting point is the Wikipedia page.
Some sites, such as at this link, discuss the mathematical or combinatorial aspects of the puzzles and/or techniques for solving them.
Finally, some sites present puzzles of various degrees of difficulty. The customers of this web site probably have a greater interest in sites that specialize in difficult puzzles, such as fiendishsudoku.
The most popular reader for PDF files is that of Adobe.
Other readers, such as Foxit, are available as well.