Download and install LINQPad, it works for SQL Server, MySQL, SQLite and also SDF (SQL CE 4.0).
Steps for open SDF Files:
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Click Add Connection
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Select Build data context automatically and Default (LINQ to SQL), then Next.
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Under Provider choose SQL CE 4.0.
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Under Database with Attach database file selected, choose Browse to select your .sdf file.
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Click OK.