Recommend an element use to fill bottles that contain ancient paper. the element should be a gas at room temperature, should be denser than helium, and should not easily react with other elements.

Okay so we are given these requirements:

element which can be used to stuff bottles that enclose ancient paper
must be a gas at room temperature
must be denser than helium
must not react with other elements

The only element that comes into my mind is:

Argon

a plasma is a warm ionized gasoline consisting of approximately equal numbers of definitely charged ions and negatively charged electrons. the characteristics of plasmas are notably unique from the ones of ordinary neutral gases in order that plasmas are considered a awesome "fourth kingdom of remember."