Links to download sql server documentation:
SQL Server 2012:
Product Documentation for Microsoft SQL Server 2012
SQL Server 2008 r2:
Microsoft SQL Server 2008 Books Online
SQL Server 2008:
SQL Server 2008 R2 Books Online
Microsoft SQL Server Protocol Documentation in PDF
SQL Server 2005:
SQL Server 2005 Books Online
Note: Currently books online is not available as pdf download.
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