A Census record for London from 1901 includes “Folder Work” as the occupation of a 15-year-old boy. (What is possibly the same individual a decade later was occupied as a “Malsters Labourer”.):
The above is a snip from UK Census Online with further details behind a paywall. I have not found a version on FreeCEN or IGI.
There are fairly comprehensive listings of Victorian occupations at London Census 1891 Transcription Blog, at Obscure Old English Census Occupations, and elsewhere, but I have not found a description for “Folder Work”.
My guess is that it may be the folding of laundry after washing and ironing as I is for Ironer by Amanda Wilkinson for example makes the point that laundry work of various kinds in 1901 kept many occupied:
over 180,000 women were recorded in the census as working in laundries as washerwomen, ironers and manglers
with a hint that though the emphasis was heavily on female labour, some males were employed:
Consequently at the Lower Croft boys and women were dispensed with, and the light labour given to old men and cripples
However, folding laundry Is not what I would have expected a 15-year old boy to be occupied with in 1901 so I would appreciate an authoritative answer to replace my speculation as to the meaning of “Folder Work”.