It looks like that Left Join is essentially an inner join... you are filtering the results back down AFTER you are first putting together the bigger recordset. Make this

tbl_documents AS a

INNER JOIN dbo.tbl_documentdetails AS b --WITH(NOLOCK)

ON a.invoiceid = b.invoiceid

AND a.orderId = b.orderid

AND a.billid = b.billid

AND b.srnumber BETWEEN 1 AND 100

Having Clustered indexes on the ID columns are a good bet as well.