Will this work?Your mileage may vary because you have not stated your VERSION of PostgreSQL or your O/S.
UPDATE your_table
SET your_column = REPLACE (your_column, 'HVACR1114_LAB_13A.pdf', '&file=HVACR1114_LAB_13A.pdf')
WHERE <whatever_expression is needed>;
What part of your solution is version or O/S dependent?
David J.