WEST LINN, Oregon — A 135-year-old paper mill in West Linn plans to lay off 158 workers this week and may shut down permanently at the end of the month if it cannot find a buyer or investor. A permanent shutdown would trigger the layoff of an additional 65 workers, according to a notice Willamette Falls Paper Co. sent to state and city officials Tuesday. “While it is hoped that this will be a temporary layoff and a buyer will want to continue the plant’s operations and hire our employees, if Willamette Falls Paper Company is unable to raise capital or sell its assets, the mass layoff will be permanent,” company President Brian Konen wrote. Workers received notice of their pending layoff Tuesday. They will be out of work indefinitely beginning Friday, when the mill will cease operations. Konen said the company will make a final decision on the plant’s future by Aug. 30.