Thank you guys for the answers.
I've been doing some research on the subject and it seems that a lot of processes can magnetize a knife, like machining, grinding, sharpening, polishing, welding, laser printing, among others.
I also saw companies offering demagnetizing services for surgical instruments to avoid contamination by eventual metallic particles attached to the instruments, being that one of the disadvantages of magnetized blades.
I wonder if carrying a magnetized folder in your pocket or holding it for long periods of time can have any negative health effects.