Fujitsu FI-5900C Operator's Manual page 36

The adjustment is done by using the Operator Panel on the scanner.
Function
Hopper Height
Paper Thickness
Thin Thick
Send to Scan
When you press the button, the hopper is raised one step higher.(Low Middle High)
When pressing the button, the hopper is lowered one step lower.(High Middle Low)
Immediately after the scanner is turned on, the Hopper is initially set to Low.
HINT
16
2.5 Setting the Hopper Height