BenQ doesn't post drivers according to the model # on the tag at the rear of the unit. They post them according to the model # listed on the top of the scanner.
eg: 3000U, 5450U,
7400U, 7400UT, 310U, 340U, 5300U, 320U, 610U, 620U, 640U, 3300U, 5000U, 6400UT, 1240UT, etc.
Determine which model you have then click the link below and select your model from the list presented.
Acer/BenQ driver for model ScanWhiz 5450U