The free-energy change (deltaG) for the oxidation of the sugar molecules in a sheet of paper into CO2 and H2O is large and negative (deltaG = -2833 kJ/mol). Paper is stable at room temperature in the presence of oxygen (O2) because it requires to start the reaction.

activation energy to start the reaction.

Explanation:

The free energy change ΔG of the oxidation of sugar molecules in a sheet of paper in presence of ait has negative value. So the reaction will be spontaneous but it requires initial activation energy which it is not getting at room temperature . So it requires its initial temperature to reach its flash point to attain activation energy . Once it gets activation energy , the further reaction will be automatic  due to negative value of ΔG.