Downhole pressure equalizer characterized in that it comprises: - an elongated housing comprising a piston compartment having a first end provided in fluid communication with an outside of the elongated housing and a second end provided in fluid communication with a process chamber of a well tool; - a piston device slidingly and sealingly engaged within the piston compartment, wherein the piston device is separating the piston compartment into a first sub-compartment between the piston device and the first end and a second sub-compartment between the piston device and the second end; wherein the piston device comprises: - an outer piston section slidingly and sealingly engaged with the piston compartment; wherein the outer piston section comprises a sleeve having a through bore; - an inner piston section slidingly and sealingly engaged within the through bore of the outer piston section; wherein the downhole pressure equalizer is configured to be in one of the following states: - a first state, in which the pressure difference between the fluid pressure in the first sub-compartment and the fluid pressure in the second sub-compartment causes the outer piston section and the inner piston section to move towards the second end of the piston compartment; - a second state, in which the pressure difference between the fluid pressure in the first sub-compartment and the fluid pressure in the second sub-compartment causes the inner piston section to move towards the first end of the piston compartment while the outer piston section is kept stationary. - an elongated housing comprising a piston compartment having a first end provided in fluid communication with an outside of the elongated housing and a second end provided in fluid communication with a process chamber of a well tool; - a piston device slidingly and sealingly engaged within the piston compartment, wherein the piston device is separating the piston compartment into a first sub-compartment between the piston device and the first end and a second sub-compartment between the piston device and the second end; wherein the piston device comprises: - an outer piston section slidingly and sealingly engaged with the piston compartment; wherein the outer piston section comprises a sleeve having a through bore; - an inner piston section slidingly and sealingly engaged within the through bore of the outer piston section; - an outer piston section slidingly and sealingly engaged with the piston compartment; wherein the outer piston section comprises a sleeve having a through bore; - an inner piston section slidingly and sealingly engaged within the through bore of the outer piston section; wherein the downhole pressure equalizer is configured to be in one of the following states: - a first state, in which the pressure difference between the fluid pressure in the first sub-compartment and the fluid pressure in the second sub-compartment causes the outer piston section and the inner piston section to move towards the second end of the piston compartment; - a second state, in which the pressure difference between the fluid pressure in the first sub-compartment and the fluid pressure in the second sub-compartment causes the inner piston section to move towards the first end of the piston compartment while the outer piston section is kept stationary. - a first state, in which the pressure difference between the fluid pressure in the first sub-compartment and the fluid pressure in the second sub-compartment causes the outer piston section and the inner piston section to move towards the second end of the piston compartment; - a second state, in which the pressure difference between the fluid pressure in the first sub-compartment and the fluid pressure in the second sub-compartment causes the inner piston section to move towards the first end of the piston compartment while the outer piston section is kept stationary.