You should alter your parameter code to include the SQL wildcard.
com.Parameters.Add("@DocumentCode", SqlDbType.NVarChar).Value = Passvalue.ToString().Trim() + "%";
The reason that your SQL wasn't working is that this part of the query
DocumentCode LIKE @DocumentCode + '%'
Becomes this SQL
DocumentCode LIKE 'DOCCODE' + '%'
The SQL becomes a string concatenation, not a value with a wildcard.