A sheet folding apparatus for folding sheets comprising a curved conveyance path along which the sheets are moved successively in first and second opposite directions, the curved conveyance path being such that the sheets cannot naturally register to reach a folding position by simple effect of gravity, wherein to register the sheets before to fold them transport rollers having loaded the sheets in the first direction moves back in the second opposite direction for compressing the sheets against an adjustable low stop member to generate an elastic deformation of the sheets sufficient to compensate friction in the curved conveyance path and automatically register the sheets.