Links

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.

Puzzle Sites

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.

PDF Readers

The most popular reader for PDF files is that of Adobe.

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Other readers, such as Foxit, are available as well.