Boox Color Series User Manual page 31

Lock ( Reading Password)
User can add (reading) Password to lock each file in order to protect
personal reading data；
The Password is used to open the file,but it only applies to the same
device and NoeReader. If you copy and export the file to other devices or
Apps, the Password will become invalid.
You can use the same Password as the Screen Lock. Or you can set a
different digits Password.