Try:
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand cmd = new SqlCommand("SELECT myPDFData FROM myTable", con))
{
using (SqlDataReader reader = cmd.ExecuteReader())
{
while (reader.Read())
{
string myPDFData = (string) reader["myPDFData"];
File.WriteAllText(@"D:\Temp\mydata.PDF", myPDFData);
}
}
}
}