PhpMyAdmin has a nice feature that creates a PDF file with the database schemas and the database relations.
It's quite cool. You see the whole database each related field is connected to each other with an arrow in the direction of the relation. Also primary keys are highlighted. You can customize the position of the tables too (though it might take a minute or two to get the hang of it using the DHTML editor, if you have a large amount of tables).
If you have the pma db and tables setup correctly, PhpMyAdmin will remember related fields and already generated PDFs.